If you look at the constellation Leo, you won’t see Segue 1. It’s the dimmest galaxy in the observable universe; it also happens to be a dwarf galaxy and, being 23 kiloparsecs away, it isn’t really close enough to get a good look at. For astronomers, studying Segue 1 is an awful lot like conducting an archaeological dig blindfolded. In mittens.
But once you record an image of the right section of Leo and take out the pieces that aren’t Segue 1, as a team of scientists led by Yale astronomer Marla Geha did a few years ago, you’ll find that Segue 1 has some interesting characteristics that may tell us something about the structure of the universe. Geha talks a bit about near-field cosmology, how one can find galaxies in the deep field, why Segue 1 (which she calls her favorite galaxy) is important, and how scientists plan to study it further, here:
Segue 1 is, to begin with, one of the oldest galaxies in the observable universe, it’s not producing any new stars, and the few hundred stars it does have are just sort of sitting there letting their remaining energy burn out. But the point Geha makes that I find especially interesting is the galaxy’s mass-to-luminosity (M/L) ratio: 3,400, which means that the galaxy is 3,400 times more massive than it is bright. This strongly indicates that Segue 1, in addition to being very old and very dead (at least by the standard of luminosity), is made up almost entirely of dark matter.
It is, in other words, the very cold husk of a very ancient dead galaxy, dim and tiny, that has been orbiting our own Milky Way for a human eternity—and its considerable mass is made up of matter we can’t currently see, hear, or fully explain. It is simultaneously an embryo, a fossil, and a ghost.