We're Not 100% Sure They're Black Hole or a Dark Star

We can be surprised by the universe in two different ways. One way is to come up with a new idea for how the universe works and then go out and search for those things. Einstein's general relativity predicted black holes before we even saw them, it suggested these should be a real thing. We've never been close enough to one and never seen the heart of one to be sure that there's really a general relativistic singularity. There's definitely something they are very compact and very massive, very much distorting space and time.