Q&A: Dark Matter

Q:
What if we viewed dark matter as easily disrupted and smooth flowing through the universe? Could it be possible that it could collide with another cloud of dark matter or become disrupted by some type of gravitational pull, if we mirrored this to events in our atmosphere, but instead of high and low pressure, different gravitational pulls, something that might violently react? Or some other element. With the motions of dark matter and the similar attributes of a black hole (void of light) and the fact that most scientists believe that dark matter is connected to black holes would that not indicate at some point they must have crossed paths or been composed of like substances?

A:
It does seem that at dark matter and black holes should interact. If there are black holes distributed in some way throughout galaxies (of smaller mass than the massive black hole in the center of most galaxies) as does seem to be the case, then they must at some point come into contact with the huge halo of dark matter that surrounds a galaxy like our own.

What happens when they cross paths? It depends - on what dark matter actually is. We could imagine that dark matter particles would fall into the black hole, because they seem to interact gravitationally as any other massive particle. The black hole would gain mass and would perhaps attract more dark matter, and at a certain point it would have pulled in the nearest particles and the rest of the dark matter would be too far away to be affected. We might test this if at some point we could monitor the growth of black holes near the halo, which we cannot do at this time (and in fact measurements like this seem a long way off!).

Keep in mind that just because two masses interact, such as black holes and dark matter, does not mean they are the same thing. The only inference that can be made from the fact that two bodies interact via gravity is that they both have mass. Nothing more. Imagine a girl standing across from a wall, hitting a tennis ball which bounces off the wall and comes back to her so that she can hit is again. The girl and the wall interact via the ball, but this does not mean this girl and the wall are the same.

The key idea is that we don't know what dark matter is. It could be an indication that the laws of gravity need to be modified in some way, even though this seems unlikely.

As far as dark matter flowing smoothly through the universe, this has not been shown to be true. The evidence shows that dark matter is concentrated in galaxies and clusters of galaxies. Within this concentration, dark matter could consist of a cloud of particles. Dark energy however, does seem to be a smoothly distributed energy spread throughout the Universe.

More information:
http://chandra.harvard.edu/photo/2004/darkenergy/
The links at the bottom will take you to an interview with a prominent cosmologist who explains what dark energy means and an article on measuring dark energy in the Universe.