galaxies

Twenty years ago, astronomers couldn’t find enough satellite galaxies orbiting the Milky Way. Now there seem to be too many. Some information seems to be missing. A possible solution is that many of these galaxies are dwarfs formed by dark matter: Most cosmologists believe that dark-matter particles are “cold,” meaning that they move slowly. Because of this, they can coalesce into numerous tiny halos, providing scores of places where dwarf galaxies can form. But “warm” or “hot” dark matter, which by definition moves faster, cannot coalesce so easily. In fact, hot particles wouldn’t be able to form mini-halos at all. So the sheer existence of these small galaxies is a sign that warm dark matter is likely not at play. Read More ›