Expert: Courtney Seligman - 8/11/2009

Question
What is dark matter and what it is doing in space? If there is no light in a room, will the dark matter occupy the room? I got confused..! Explain me please.

Answer
We don't know what dark matter is. As far as dark matter in galaxies is concerned, it could be normal matter in some hard-to-observe form (e.g., dead stars, or gases so thinly spread out they have little interaction with light), or some kind of strange matter that we have never observed, have no idea what it is like, and might not recognize as matter even if we could observe it. (For a thorough discussion of some of the possibilities, refer to Dark Matter in Galaxies, on my website, at http://cseligman.com/text/galaxies/dark.htm )

However, let's say that dark matter is something which is all around us, and we just can't tell it's there. If so, it is in the vast spaces between the stars that the mass of dark matter becomes significant. In regions as small as our solar system, the amount of dark matter is so close to zero that we can't even tell if it's there.

To see how this works, suppose that we consider how much dark matter there is -- namely, about ten times as much mass as is in the visible stars. Now take a star like the Sun. It has a density of about 1.4 times the density of water; but it is less than a million miles in diameter, or less than 1/2 of 1% of the size of our orbit. If we expanded it to the size of our orbit (which it will do when it becomes a red giant, 7 or 8 billion years from now), it would be over 200 times larger in diameter, and about 10 million times larger in volume. That means it would have an average density 7 million times less than that of water, and 7 thousand times less than the air at the surface of the Earth.

Now let's expand the Sun still further -- not just to the size of our orbit, but halfway to the next star, which would make it more than a hundred thousand times larger in diameter than our orbit, and give it a volume more than a million billion times more than if it were the size of our orbit. In other words, if we spread out the mass of stars like the Sun throughout the region between the stars, the average density of the material would be thousands of millions of billions of times less than the density of air. Given that very low density, even increasing it by a factor of ten (which is what we're talking about, for dark matter), there is still essentially nothing there.

In other words, even if dark matter 'fills' all of space, even passing right through us, our planet, our solar system, etc, its density would be many hundreds of millions of billions of times less dense than air. So an empty room 'full of' dark matter would still be a VERY empty room. And in fact, if the entire solar system were 'full of' dark matter at this same low density, the total mass of dark matter in the solar system would be billions of times too small to notice.

So no, a dark room is not full of dark matter. It just doesn't have any light in it. And even if it were 'full of' dark matter, the amount involved would be so small that it would be next to impossible to detect, even if we knew what to look for, which we don't. And it could be that there wouldn't be any dark matter in the room at all, because it might not be spread through all of space, but concentrated in a relatively few objects of unknown nature, out between the stars.