Could dark matter particles be unstable?

Dark matter is actually matter. Real matter, just that we can not observe it.

Part of it is actually cold hydrogen, and even cold dust.

Other parts of it are actually different particles that may decay. But that does not mean formation of new Hydrogen. To form Hydrogen you need both a proton and an electron to get bound together, and this implies they meeting, which is very difficult in low density regions like those of extragalactic dark matter.