If you don’t know anything else about the Milky Way galaxy, the galaxy our solar system resides in, you probably know these two things:
- Although it’s not a particularly large galaxy by cosmic standards, it’s freaking huge by any standards we’re familiar with: 100,000 light years in diameter, containing about 300 billion solar systems. None of our unmanned probes are likely to ever leave it. We may as well pretend the Milky Way is the entire universe, really, because the only way we can interact with galaxies outside of it is by observing what they were like in the distant past.
- It’s also really fast by human standards, moving through space at a speed somewhere between 130km/second and 600km/second (290,000 to 1.3 million miles per hour).
Which isn’t to say that some of these celestial objects aren’t a little bit spooky. Take Smith’s Cloud, for example. About a tenth as wide as the Milky Way and headed for the Milky Way’s Perseus arm in about 27 million years, it stands out for one very simple reason: it has already collided with the Milky Way at least once 70 million years ago, and it came out unscathed.
Now, armed with new data from the NRAO’s Green Bank Telescope, scientists are beginning to figure out the secret of Smith’s Cloud’s invulnerability: it’s most likely surrounded by a shield of dark matter. As National Geographic’s Nadia Drake explains:
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).
So what else owes its cohesiveness to dark matter? And where did this stuff come from, anyway? It might be a long time before we know the answers to these questions. And Smith’s Cloud and our beloved Milky Way keep coasting through the universe, just the same.