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Giant collision created galaxies devoid of dark matter
Two galaxies that are curiously lacking in dark matter — the most abundant matter in the Universe — might have formed when a collision between dwarf galaxies separated ordinary matter from its dark counterpart.
When we clap our hands together, the electrons in each hand repel those in the other electromagnetically, preventing our hands from moving through each other. But what would happen if our hands contained some dark matter, which is thought to interact mainly gravitationally, but not electromagnetically? The clap would separate each hand into ordinary matter, which would stop with the collision, and dark matter, which would pass through the other hand. Fortunately for us, our bodies comprise only ordinary matter, but such events can occur on a cosmic scale when two galaxies collide. Writing in Nature, van Dokkum et al.1 propose that such a collision might have created two ultra-diffuse (low-luminosity) galaxies that have no dark matter.