Big fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so, ad infinitum.
From "Siphonaptera," Augustus de Morgan
When you look at a big galaxy like the Milky Way, you see it has a lot of components. There's a central bulge of old stars, a flat disk of stars (usually sporting spiral arms) around it, a halo of old stars surrounding that, a dark matter halo which we can't see directly but which has profound gravitational effects due to its huge mass, and a coterie of smaller dwarf galaxies and clusters of stars orbiting it.
It's those last two which have recently come to more attention by astronomers. We think that dark matter clumped together in the early Universe, and 'normal' matter (the stuff that makes up stars and planets and you) was drawn into it gravitationally, forming galaxies. Galaxies themselves wound up getting huge halos of dark matter, including galaxies that are very large (like ours) down to far smaller ones.
In fact, one prediction is that the dark matter halos around galaxies are 'scale invariant,' a fancy term that means that a halo surrounding a big galaxy looks very ...