We are located about 30,000 light years out from the galactic center (1.8E17 miles), and the galaxy goes round every 200,000,000 years. From the rotational rate and diameter I calculate that we’re moving at roughly 1,000,000,000 miles/year or 100,000 mph — not a bad speed to expect to come from random variation of the gas molecule speeds. Maxwell averaging should reduce the speed to 2000 mph at most, I’d think.

Even more interesting, the rotation speed suggests the galaxy’s gone around about 50 times since it condensed. That’s an awful lot of turns for our galactic arms to retain stable; you’d expect that the outer parts of the arms would have rotated far fewer times, perhaps only once, while the inner parts would rotate perhaps 1000 times. After a billion years, you’d expect the arms to be gone. The going explanation is dark matter, matter we can’t see.

After bugging astrophysicists for a few years, I’ve come to believe that many of their models (MACHOs, WIMPs) don’t make much sense. I’ve come to model the distribution of dark matter on my own, as a particular distribution gas cloud of light particles. There is only one distribution that will result in the galaxy rotating as a unit — can you figure out what that is? Not that I now know what dark matter is, but at least I think I know where it is. Now all we need to do is find the missing matter. As a challenge, see if you can calculate the distribution of dark matter that would result in the galaxy rotating as a unit.

— Robert Buxbaum