physics question : dark matter?

Question

In 1930s that our own Milky Way galaxy behaved as if it contained more matter than could be seen with telescopes. This puzzled astronomers, astro-physicists. So, this non-luminous matter became known as "dark matter" and we now know that over 90% of the matter in the entire universe is dark.
What is this dark matter made of? And what happens if dark matter and regular matter collide or put together in laboratory, for instance? When people talk of dark matter, they think of the milky way or stars, but what about on earth? is there evidence of dark matter on earth? Is there an active search for it?

Answer 1

Dark matter is a mysterious form of matter which exerts gravitational effects on ordinary matter but cannot be observed via any other means. While conventional matter-clouds can be detected through their stellar activity, light scattering, radio emissions, and other means, dark matter clouds are invisible to the vast majority of our instruments. Yet they constitute 90% - 99% of the matter in the universe.

In the 30s, astronomers Fritz Zwicky and Sinclair Smith performed an analysis of the velocity of galaxies within the large galactic clusters Virgo and Coma. They discovered that all galaxies were moving between ten and a hundred times faster than they should be, given the estimates based on observed stellar density. Something unseen was generating additional gravity.

Although Zwicky and Smith's initial observations seemed to provide strong evidence for dark matter, not all cosmologists were convinced. Because these galactic clusters were so far away, it was hard to accurately measure the independent velocities of each galaxy on a timescale of mere decades. Stronger evidence came in the 70s when scientists such as Rubin, Freeman, and Peebles began to analyze the rotation curves of galaxies. Stars in spiral galaxies, such as our very own, were moving faster around the galactic core than their observable mass would indicate.

It was eventually discovered that all galaxies contain dark matter halos that stretch far beyond the bounds of the galaxy indicated through a telescope. These halos are said to be made up of MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles). MACHOs are theorized to mostly be black holes and brown dwarfs (burnt-out stars). They are made up of conventional, or baryonic matter - the atoms we are familiar with, only highly compressed. WIMPs are said to be non-baryonic forms of matter, weakly interacting particles which move at relativistic speeds. The most likely candidate for WIMPs are the neutrino and its cousins.

Answer 2

Actually nobody knows what this dark matter consists of. There are some good candidates like Higgs particles, a theoretically postulated, very heavy particle that will be searched by the collider in Geneva, Switzerland. However the lists of past cadidates being disproved is pretty long.

The trouble with dark matter is that it apparently interacts only gravitationally with normal matter. By applying celestial dynamics one an figure out where a lot of dark matter must be (outer parts of galaxies for example), but if we look there with telescopes we actually "see" nothing. So this matter neither interacts with photons, magnetic fields and all the particles emitted from a galaxy.

I think the one being able to show what dark matter consists of will win the noble prize immediately. So there is a lot if research going on in this direction.

Answer 3

Nobody knows for sure what it is, or even IF "dark matter" exists. Some think its neutrinos, which are thought by others to be massless, so how can they have any gravitational effect? Others think it's left over baryon "junk" from the Big Bang.
I think it might wind up being folds in what we are calling space-time, where the other 11 dimensions of superstring theory are all curled up. Or it might be something else entirely, such as a macroscopic property of gravity, much in the same fashion as lightning is a manifestation of figuring out electrons....

Answer 4

'know' is too strong a word. You're right, science couldn't make sense of the motion of the universe based only on observable matter. Then, someone offered the hypothesis that the universe contained matter with gravitational mass but which did not interact with electromagnetic fields. Then scientists got down to the hard work of figuring out just how much of this 'fudge factor' to add to the universe so the motion would make sense. They've now done that well enough to say that, if dark matter exists with these properties and this distribution, the kinematics of the universe would make sense. They haven't found anything to refute the hypothesis. Evidence is beginning to appear which seems to support it. It's much harder to detect if it's hidden among traditional matter than if it's isolated. It is considered to be of extremely low density, but rather uniformly distributed (unlike traditional matter).

Answer 5

I know very little of the details behind the observations of dark matter, so I cannot give you any foundation for my opinion, but I have a gut feeling that in the end dark matter will turn out not to be something real but rather an artifact of a broken theory of gravitation.

People who know more will probably say I don't know what I'm talking about, and they may well be correct.