Weirdly, the biggest part of a galaxy is the hardest thing to see in it.
Or, more accurately, around it. I'm talking about a galaxy's halo, the huge spherical volume surrounding a galaxy, typically made up mostly of gas (there are some stars in it but not many… and there's also a dark matter halo, but we'll skip that for this discussion).
When you think “spiral galaxy” you probably picture a disk of gas and stars, maybe with a bulge of stars in the middle. That's fine, but the halo is so huge it positively dwarfs that inner part. A disk might be 100,000 light years across, but the halo can be ten times that. More.
It's so faint it's hard to see, though. Even nearby galaxies have halos so faint you can't really see them directly. However, astronomers are clever, and have found ways to measure them indirectly. Using such a technique, aided by observations with Hubble Space Telescope, new research shows that the nearby Andromeda galaxy has a layered halo, two nested spherical shells, with the outer one reaching out to a staggering two million light years.
Our own Milky Way has a halo, too, and it reaches ...