Much about the outer solar system is still a mystery.
That's not a huge surprise, because it's very far away. Neptune, the outermost large planet, is about 4.5 billion kilometers out from the Sun, fully 30 times farther out than the Earth. And that's the inner edge of the outer solar system, so we're talking distances that are truly vast. Even big objects out there are hard to see.
It had been predicted for decades that largish icy/rocky bodies would be out there past Neptune, with Pluto just being the biggest of them. Pluto was discovered in 1930, but the next Kuiper Belt Object (KBO), as they're called, wasn't found until 1992. Now we know of over 1,000, but even the biggest are barely more than dots in our largest telescopes. That makes them hard to study.
… but not impossible. In fact, we do know many interesting things about them. For example, many have satellites. The six largest KBOs all have satellites, almost all of which are much smaller than the parent body. It's also known that many smaller KBOs are binaries, pairs of roughly equal-sized objects. Likely these smaller binaries formed as pairs, or formed as separate objects ...