The misplaced modifier in the sentence is "Though they are incredibly massive." This phrase is meant to describe black holes, but it is placed in a way that makes it seem like it is describing astronomers instead. To correct the sentence and avoid the dangling modifier, you can rephrase it like this:
"Astronomers cannot actually see black holes, though these cosmic objects are incredibly massive; instead, they look for radiation from fast-moving gas around the black hole."
By moving the modifier closer to "black holes," we eliminate the ambiguity and clarify the intended meaning of the sentence.
