By now, nearly everyone has heard of dark matter. If we ignore dark energy, then dark matter is the stuff that comprises the majority of our universe. However, it is not stuff, as we know stuff. We are mostly familiar with stuff like atoms and molecules that arrange to form the tangible substances around us. As such, it is baryonic. Dark matter, presumably, is not. If the particle physicists are right, dark matter does not interact with us, or any normal matter, in any normal way, except gravitationally. Particle physicists suspect that dark matter is actually WIMPs (weakly interacting massive particles). Some claim that dark matter may exhibit charge as well as gravitation, helping to better explain dwarf galaxies.
This may seem a little strange, and it is. It could well be wrong, too. But evidence is growing, only not quite as quickly the wild imaginings of scientists. However, we do know with certainty that something strange is afoot. Vera Rubins was the first person to empirically disclose it. If you remember, she was observing the huge galaxies out there, where you see lots of stars clumped together, turning like giant wheels. The strange thing is, there is not enough substance in galaxies to hold them together in their shapes as they turn. They should be flying apart. But they don’t. And that’s crazy.
More evidence for invisible dark matter comes from subtle observations. Einstein said that any object with mass will warp the space it occupies. That’s what makes gravity, according to him. It’s not really a force. You just slide down that warping, toward the center of the warp. Or rather, you slide toward the center of mass, like the center of the Earth. Or the sun. We orbit the sun because we’re falling toward it, but our centrifugal force of motion counters it, so we keep our distance. (I know, it’s more accurate to talk about inertia…)
But there are other implications to the warping of space. If space actually warped, it should change the path light travels, which is normally in a straight line. People trying to prove Einstein wrong tested this by looking at the stars visible around the sun. They found that they could see stars that should have been hidden behind the sun, but were visible anyway. This is called gravitational lensing, where light appears bent around massive objects, so that you can, amongst other things, see what is behind them. These people ended up proving Einstein right. It appears that objects with mass actually do warp space.
As a side note, this is a huge problem for particle physicists in quantum mechanics. They like to think of gravity as being a force-like thing that is “communicated” between objects with mass through a particle called a graviton. Even if they discover a graviton, it’s a huge leap to tie it back into whatever substance space is made of, so that they can also warp it, as we know that it must.
Anyway, the larger the mass of something, the more it warps the surrounding space. Galaxies have a lot more mass than our sun. And this is where we find another strong case for dark matter. We have observed gravitational lensing happening around galaxies. And the lens strength is far too strong to be coming from the mass of only the visible parts of the galaxy. There must be a considerably larger amount of material in the galaxies to account for the degree of warping we observe.
Another compelling piece of evidence is found in observations of cluster MACS J0025.4-1222. This is a composite image, created by combining observations from the Chandra X-ray Observatory and Hubble. This is apparently a collision of two clusters. The red part is Chandra showing the glowing gases of the two clusters being excited as they collide. The blue part is the implied dark matter distribution based upon gravitational lensing effects observed by Hubble. It appears that the dark matter passed through the collision unaffected by any matter, while the baryonic matter interacts in the center. The evidence is compelling. But it is not definitive, as many people portray it.
There are also a couple mathematical arguments that suggest dark matter, but they confuse me. I won’t even pretend to understand them. I don’t trust them, either. String theorists, particularly those with a passion for supersymmetry, are too comfortable building houses of cards, and cheating their way out of death. I’ll stick with the experimental physicists.
So we know something massive needs to exist in every galaxy — a massive thing that we cannot detect. For a long while, many scientists thought it might be brown dwarf stars (very dim) and black holes which might account for all this extra mass. We wouldn’t be able to see them. However, despite having thoroughly watched, we have never seen something unexpectedly blot out any objects behind it, as should occur with large objects. So it seems the invisible stuff is something altogether new.
If dark matter is, indeed, non-baryonic, how can we know for certain that it exists? Well, if dark matter is actually WIMPs, as the theoretical physicists suspect, it would exist as a massive invisible cloud that fairly evenly surrounds galaxies (and us!). Dark matter would annihilate with itself whenever it came in contact with other dark matter, too, possibly creating an electron and an anti-matter electron (positron) in the process, with a burst of gamma energy. In this case, dark matter would likely be neutralinos, which is yet another subatomic particle in the particle physicist’s menagerie.
It is possible some evidence of this will be forthcoming. Remember all those space observatories I wrote about a while ago? One of them was WMAP and its job was to measure and map the background radiation of the entire universe. As it was looking around, it also had to look through the center of our own Milky Way. It was very, very bright. Too bright. And there are many who suspect this might be a result of the energy released by dark matter self-annihilation. And though it’s not saying much with theorists, it does say something — they can make the math work, too.
Dark matter is most certainly strange. Excluding dark energy, nearly 85% of the stuff around us is actually invisible dark matter, whatever it might turn out to be. We would have to ignore a lot of evidence to discount the existence of dark matter — or perhaps we are just not taking something very fundamental into account. And some galaxies are even stranger. For example, the dwarf galaxy Segue 1, which orbits our own Milky Way, looks to be composed of over 98% dark matter. It is a large turning cluster of invisibility, with just a few stars rotating along with it.
So here you go. Hopefully it will help you get into the Halloween spirit, knowing that there actually may be dark, invisible things literally surrounding us. It’s certainly looking more and more true.