Where has all the dark matter gone? Has it disappeared down all those black holes? Does it really exist?

Question

I am a complete layman in these matters

Answer 1

The dark matter hasn't gone anywhere. The problem is we can't see it directly because it doesn't or reflect any light (hence the name!). It is defninitely there, but we don't know what form it takes - is it normal matter that just happens to be really really dark (dark dust, very faint brown dwarfs, isolated black holes)? Or is it some form of weird, exotic matter that we have yet to discover (I know that sounds far-fetched on the surface, but it actually isn't).

We suspect that dark matter exists for various reasons. One is by observing how spiral galaxies rotate. By watching any object orbit around another (like our Sun or other stars orbiting around the center of our galaxy) we can calculate the mass of the thing being orbitted. But with all spiral galaxies (and others, too) the mass we calculate is far greater than the mass of all the stuff (stars, gas, dust) that we can see - therefore there must be stuff in our galaxy that we can't see, but that still has a significant gravitational effect.

We also see galaxies orbiting each other. Here again, but watching them we can calculate how much mass there is, since mass determines gravity. If we only had the mass of the visible stuff inside and outside the galaxies, those galaxy clusters should break apart - the gravitational force from all the visible stuff isn't strong enough to hold those clusters together. But those clusters do stay together, so there must be more dark matter there between the galaxies.

Now you may just be asking yourself "maybe gravity doesn't work the same on the galactic scale as it does here on Earth", which is a question scientists have asked, as well. But recent evidence suggests that that is not the case - that we really do need dark matter to explain our observations.

It gets even weirder - we now think there's something called "dark energy" that is causing our Universe to epand at a faster rate, instead of being slowed down by gravity the way we used to expect.

There are quite a few good books about this stuff written for laymen - just puruse your local book store in the science section. And if it all seems too confusing, don't worry - it's confusing to the scientists, too! As I tell my students, if it all seems to make sense, you're not paying attention!

Answer 2

To add to the above.

The current best idea about Dark Matter is that it consists of particles called "axions" which have the right properties and are thought to have been created in large quantities in the Big Bang.

As far as the measurements of gravity by rotation curves of galaxies and galaxy trajectories in clusters of galaxies, the Dark Matter could be anything that has mass, and doesn't interact with normal matter very much. It could, for example, be made of rocks (or "bricks" as some jokers have suggested), or smallish Black Holes that don't emit much radiation from an accretion disk. The problem with making the Dark Matter out of "normal" matter (that is to say baryonic matter like us, the Sun and the Earth) is that there is so much of it, it would screw up nucleosynthesis in the first few minutes of the Big Bang. The amount of Helium and Duterium in the Universe is about right, given the amount of Hydrogen, provided the Dark Matter is not made of baryonic matter. This implies that the Dark Matter is not baryonic matter, and is therefore "exotic", such as Axions.

The "wierd gravity" theory of Dark Matter has not gotten very far theoretically. However, the evidence for "Dark Energy", which is manifestly a kind of wierd gravity, makes a potential unified solution to both problems more attractive. Theories abound, but the evidence is, as yet, lacking.

Answer 3

basically Einsteins gravitational equations state that there should be a lot more mass in the universe that we can measure - the gravitational bending of light by mass that is unmeasureable.

because Dark Matter does not reflect any electromagnetic radiation (radio-waves to lightwaves, to x-rays, etc) it is impossible to know it is there, although it's effect on light is measureable.

Answer 4

Dark matter was invented to explain the evolution of spiral galaxys.
Neutons law of orbital systems does not apply in the case of a galaxy.
Dark matter is not needed
A black hole is a theoretical entity that does not and can not exist!!

Answer 5

There are many deep mysteries in Physics. It is the most spiritual of the sciences. In this case, I believe we are moving into the territory of dimensionality and time. In short, there are parallel dimensions (realities) that are connected like layers of a cake. Dark matter is part of this connective tissue in the space time continuum.