What are halo substructures?

Halo substructures are like hidden islands inside a big, fluffy cloud.

Imagine you have a giant marshmallow, that's like a halo, which is a big group of stars and space stuff around a galaxy. Now, if you look really closely, sometimes you can see little clumps or bits inside the marshmallow, those are the substructures. They're pieces of old star groups that got caught in the halo over time.

If you bite into the marshmallow, you might find tiny crunchy bits, that's like finding halo substructures inside the big fluffy cloud. These little bits are like smaller groups of stars, hiding inside the bigger halo.

Sometimes, these little star groups come from other galaxies nearby. The big halo is kind of like a space magnet, pulling in these smaller pieces and keeping them trapped inside, just like how your favorite snack might get stuck in your teeth after you bite into something gooey!