I'm kind of reminded of the discovery of the first exoplanet. Once we found it, we started finding more and more of them across the sky. How do we know that bin

I'm kind of reminded of the discovery of the first exoplanet. Once we found it, we started finding more and more of them across the sky. How do we know that binary black holes aren't all that common? For instance, how do we know that there isn't a binary supermassive black hole at the center of the Milky Way that we can't see? For stars, binary systems are very common. How do we know that black holes aren't the same?