Ah, alright! I took it by the way their movements were all 'tacticool' like people say. Not that it's a bad thing. Good luck!

Observation of black holes interacting with matter, known laws of gravity, and physics. We don't "know' 90% of what we accept as fact in astronomy. But the idea that two black holes colliding causes their mass to combine in to one larger one is a lot more scientifically plausible than "doors to other universes". We accept as fact the most plausible explanation for astral events until proven otherwise, and right now, black holes combining together after a collision is what we accept as fact.