Wrapped in dark matter, an object known as the Smith Cloud is hurtling toward the Milky Way at more than 150 miles per second. Now 8,000 light-years away, the gassy clump of hydrogen is expected to smash into the galaxy in about 30 million years.
But that won’t be the first time this blob of gas has met our galaxy: Scientists think it’s been here before, millions of years ago.
Normally, gas clouds don’t easily survive such encounters without being torn apart; to make it through, a cloud would need to be incredibly dense. Smith’s Cloud is not.
Astronomers initially proposed that the cloud’s strong magnetic field could shield it during its galactic passage. But new observations, made using the National Radio Astronomy Observatory’s Green Bank Telescope, offer another suggestion for how the Smith Cloud endures. Like a cosmic burrito, the cloud is wrapped in dark matter — the mysterious substance thought to comprise more than 80 percent of the matter in the universe. That dark shell protects the cloud from being shredded by the Milky Way (though it does appear to have a tail, like a comet, containing material that’s being sucked up by the galaxy).
Since interstellar clouds don’t usually come gift-wrapped, scientists now suspect Smith’s Cloud could be the gassy remains of a failed dwarf galaxy that never grew up and turned on its stars.