Summary

Randal explains in depth the difference between dark matter and "ordinary" matter by noting that we humans are usually much more concerned with the latter (because that's the here-and-now everyday world) than the former. Nevertheless, it's possible that dark matter is just as important as physical matter in terms of interaction with other forces and components of the universe.

"If we were creatures made of dark matter, we would be very wrong to assume that the particles in our ordinary matter sector were all of the same type," Randall writes. "Perhaps we ordinary matter people are making a similar mistake." It's possible, according to Randall, that one type of dark matter (say, 5 percent) interacts through gravity with particles in the sphere of ordinary matter

that contribute greatly to the structure of the universe. If there is a dark electromagnetic...