The Universe is teeming with tiny galaxies, the smallest of which can be up to a hundred million times less massive in stars than our own Milky Way. The few years have seen a revolution in the discovery and characterization of these systems, including the discovery of many dozens of new ultrafaint galaxies in the Milky Way system and the characterization of Milky Way-like systems and their satellites well outside the Local Group. These faint galaxies trace the smallest observationally accessible dark matter halos, and can thus provide clues to the nature of dark matter as well as to the onset of galaxy formation. I will discuss new observational results on satellite galaxies within the Milky Way and similar systems and describe what we have learned about the dark matter halos that host the smallest galaxies. I will show how these measurements help place our own Galaxy in its broader cosmological context, and how they provide essential clues to the microphysical properties of the dark matter that dominates our Universe.
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