what is the evidence for dark matter in galaxies?

Answer 1

We look at galaxies and we add up all the matter that we can see, and with only that matter, galaxies would fall apart and stars would fly off into space...we just don't have enough matter to have the gravity to hold them together...thus the theory of dark matter came to be.

Answer 2

It basically boils down to what galaxies are doing, with the mass that is assessed from their luminosity.
A galaxy is made of billions of stars, along with gas clouds, and some black holes -- that including a supermassive one at the core.
All that mass should account for a derivable gravitational force that would need to be balanced by the rotational velocity -- the individual orbits of the stars making the galaxy around the galactic core. But observations show the repartition of that orbital velocity does not match what it should be from the gravity that is the result of the observable mass (here, observable mass includes also things like black holes and dark gas clouds, those thing can be dark, but would still have an influence on what can be seen, and are thus considered normal, observable matter). In some cases, like the first one described as far back as 1933, in thise case on a cluster of galaxy, the bodies act like if there was up to 400 times as much mass as can be observed. In such case, it is reletively easy to dismiss the presence of much more mass in the form of balck holes, as they would be swallowing everything around with huge release of X rays, or in the form of massive gas clouds, as with such mass they should collapse and condense into stars at a crazy rate -- or would already have done so long ago.
The concluision is that there is a mass of something out there (well, in here as well, it should be everywhere) something that does not interact with normal matter in any way but through gravity, and that mass should be orders of magnitude more important than the observable mass.
And it onbly gets more complicated when one factors in the fact that very distance gaalxies are receeding faster than they should with all that mass factored in, so there should be dark energy as well, which should be some sort of repulsive force and wich should be even more important in the unioverse composition that dark matter is.
Right now, estimate is the 4% of the universe is normal matter, the one that makes the earth, the stars, the black holes, etc. 22% is dark matter, so 5 times more dark matter than normal one. And the rest, 74% of the univers, would be dark energy.

Trying to figure out what that stuff is is understandly pretty hard, it like trying to guess who is behind you only through observation of the shadow they cast.

But we have some our best minds on it. So stay tuned.

Answer 3

when they measure the radial velocities of different objects in a galaxy (our sun and other stars for example) they find that there is a uniform speed all the way out to the edge. so the core isn't spinning any faster than the stars on the outer fringes are. the only way this can work is if most of the mass in a galaxy is not at the core but instead spread throughout the galaxy and surrounding it. this is the dark matter.

Answer 4

The mass of a galaxy can be estimated, as well as the rotational speed. Scientists have seen that most galaxies have a rotational speed that doesn't reconcile with their mass - for the stars in the galaxy to have stable orbits, the galaxies must be much more massive than what is observable.

Answer 5

NASA Finds Direct Proof of Dark Matter

Dark matter and normal matter have been wrenched apart by the tremendous collision of two large clusters of galaxies. The discovery, using NASA's Chandra X-ray Observatory and other telescopes, gives direct evidence for the existence of dark matter.

"This is the most energetic cosmic event, besides the Big Bang, which we know about," said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

These observations provide the strongest evidence yet that most of the matter in the universe is dark. Despite considerable evidence for dark matter, some scientists have proposed alternative theories for gravity where it is stronger on intergalactic scales than predicted by Newton and Einstein, removing the need for dark matter. However, such theories cannot explain the observed effects of this collision.

"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Doug Clowe of the University of Arizona at Tucson, and leader of the study. "These results are direct proof that dark matter exists."

In galaxy clusters, the normal matter, like the atoms that make up the stars, planets, and everything on Earth, is primarily in the form of hot gas and stars. The mass of the hot gas between the galaxies is far greater than the mass of the stars in all of the galaxies. This normal matter is bound in the cluster by the gravity of an even greater mass of dark matter. Without dark matter, which is invisible and can only be detected through its gravity, the fast-moving galaxies and the hot gas would quickly fly apart.

The team was granted more than 100 hours on the Chandra telescope to observe the galaxy cluster 1E0657-56. The cluster is also known as the bullet cluster, because it contains a spectacular bullet-shaped cloud of hundred-million-degree gas. The X-ray image shows the bullet shape is due to a wind produced by the high-speed collision of a smaller cluster with a larger one.

In addition to the Chandra observation, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to determine the location of the mass in the clusters. This was done by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity.

The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. This produced the separation of the dark and normal matter seen in the data. If hot gas was the most massive component in the clusters, as proposed by alternative gravity theories, such a separation would not have been seen. Instead, dark matter is required.

"This is the type of result that future theories will have to take into account," said Sean Carroll, a cosmologist at the University of Chicago, who was not involved with the study. "As we move forward to understand the true nature of dark matter, this new result will be impossible to ignore."

This result also gives scientists more confidence that the Newtonian gravity familiar on Earth and in the solar system also works on the huge scales of galaxy clusters.

"We've closed this loophole about gravity, and we've come closer than ever to seeing this invisible matter," Clowe said.

These results are being published in an upcoming issue of The Astrophysical Journal Letters. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center, Cambridge, Mass. For additional information and images,

Answer 6

The Physics of this is really beyond me. Apparently, there is more matter around than can be accounted for by stars, planets, and the usual things. This is the dark matter.

Answer 7

As a rule, scientists seek certainty. So it's rather unusual that for more than 70 years, many astronomers have wagered the universe is primarily made of dark matter -- a mysterious and unproven substance.

It's a bet that finally paid off, because a team of scientists working with NASA's Chandra X-ray Observatory has found direct evidence that dark matter is as real as the rings around Saturn.

The discovery cements dark matter's status as the biggest building block in the universe, while also putting to rest the nagging worries of many astronomers that they gambled wrong.

Dark matter's murky nature has always sat a bit uneasily with astronomers. "It is uncomfortable for a scientist to have to invoke something invisible and undetectable to account for 90 percent of the matter in the universe," said Maxim Markevitch, a Chandra astrophysicist and researcher with the study.

"Like determining the origin of the universe or how black holes work, dark matter is one of the holy grails of astronomy."
One of the main arguments for the existence of dark matter involves galaxies and their clusters. Galaxies whip through space at enormous speeds and are searing with hot clouds of gas. Speed and heat of galaxies should cause them to fly apart, but they don't. A leading explanation for this is that the gas and stars are held together by the gravity of dark matter. Belief in dark matter is widespread across the scientific community, but astronomers don't know what it's made of. Still, they believe it acts like it has mass and exerts gravity, yet is invisible and can't bump, touch or crash into anything.

Like determining the origin of the universe or how black holes work, dark matter is one of the holy grails of astronomy. "Little is known about it; all that the numerous searches for dark matter particles have done is rule out various hypotheses, but there have never been any 'positive' results," said Markevitch.

Doug Clowe, leader of the study, set out to see if believing in dark matter was wishful thinking or informed faith. "A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Clowe.

What's the Matter?
Clowe and astronomers with Chandra, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan telescopes all set their sights on a galaxy cluster nicknamed the "Bullet Cluster." The cluster was an exciting target because of its unique distribution of gas clouds and stars and potential for harboring dark matter.

The amount of matter, or "mass," in a galaxy is made up mostly of the gas that surrounds it. Stars, planets, moons and other objects count too, but a majority of the mass still comes from the hot, glowing clouds of hydrogen and other gases.

When the Bullet Cluster's galaxies crossed and merged together, their stars easily continued on their way unscathed. This may seem a bit perplexing, because the bright light of stars makes them appear enormous and crowded together. It would be easy to expect them to smash into each other during their cosmic commute. But the truth is, stars are actually spaced widely apart and pass harmlessly like ships on an ocean.

The gas clouds from the merging galaxies, however, found the going much tougher. As the clouds ran together, the rubbing and bumping of their gas molecules caused friction to develop. The friction slowed the clouds down, while the stars they contained kept right on moving. Before long, the galaxies slipped out of the gas clouds and into clear space.

With the galaxies in open space, Chandra scientists found dark matter hiding.

Dark Matter Weighs In
When the stars separated from the clouds, they gave astronomers a lucky chance to estimate their total mass and gravity without the hot gas. The astronomers used their telescopes and various methods to measure the mass of the galaxies.

Dark matter revealed itself when the team tried a technique called "gravitational lensing." This neat trick was inspired by Albert Einstein's prediction that stars and galaxies of high mass can bend light toward them from other sources. The amount of extra light can be calculated and tells astronomers about the size of the galaxy.

An odd thing happened in this case: the galaxies lit up far too much for their size.

Answer 8

The way that light is bent indicts a possible presents of dark matter.

Answer 9

Have you seen the stuff I cough up in the morning?