Of course, by their nature, black holes are difficult to detect.
However, some binary star systems are candidates which may show
that black holes do exist. Binary star systems are two stars rotating
amongst each other. There are a couple of cases seen, however,
where one twin star is invisible, but the visible star appears to
be rotating about something. This typically is accompanied by
a flow of material from the visible twin into what looks like a
void. This behaviour is taken as indirect evidence for the invisible
twin being a black hole.