By Harry Keller
Editor, Science Education
You may have seen the headlines already. “What is dark matter made of? Galaxy cluster collisions offer clues,” shouts the Christian Science Monitor.1
Here you have a great moment to engage students in something exciting and to use critical thinking. Dark matter is a hypothesis to explain why stars in galaxies circle around their centers faster than the observable matter says that they should, as well as other more sophisticated reasons. It’s called “dark” because it does not emit light and because you cannot see it. You might also have called it “invisible” matter, but much matter seems invisible. Besides, “dark” implies spooky, and this stuff is definitely spooky because ordinary matter moves right through it as though it isn’t there, except for gravitational effects.
The new finding just reported tell us that dark matter behaves just the same way with itself. One patch of dark matter moves right through another as though it’s not there at all (again, except for gravitational effects). This result pushes back against the most popular idea about the identity of dark matter, that it’s WIMPs — weakly interacting massive particles — because particles don’t just pass right through each other.
Two explanations come to mind. Some physicists doing the work still are seeking particles, but these other explanations could help get around that barrier. There are some truly exotic explanations going around as well, such as the existence of a “mirror universe.” It’s best to stick with the simplest ones, though. Occam’s Razor tells us so.
One explanation is that gravity just doesn’t work exactly as we expect it to, especially when dealing with very large masses spanning very large volumes of space. This is not a very popular explanation. The other is that dark matter is really energy. Because of the equivalence of energy and matter demonstrated by Einstein and captured in his famous equation, E=mc2, energy is affected by and causes gravity. Continue reading