This was cited over at Einstein. It looks like it might be of interest here.

The spiraling dance of a pair of colliding black holes should last for billions of years. Yet we’ve caught about 10 black hole collisions since 2016 — far more than we would expect. Some process must be at work to accelerate the collision process, to make black holes come together more quickly than anticipated.

https://www.quantamagazine.org/to-make-two-black-holes-collide-try-three-20190815/

This was cited over at Einstein. It looks like it might be of interest here.

The spiraling dance of a pair of colliding black holes should last for billions of years. Yet we’ve caught about 10 black hole collisions since 2016 — far more than we would expect. Some process must be at work to accelerate the collision process, to make black holes come together more quickly than anticipated.

https://www.quantamagazine.org/to-make-two-black-holes-collide-try-three-20190815/

Yes, that might be a viable explanation, but I'm not sure it will hold all the way through. It seems a bit too far fetched to me.

Why not investigate in dark matter (DM) instead?

As we perceive DM it is a form of rather thin gas of weakly interactive massive particles (WIMPs), that would accumulate around galaxies and other massive objects -- especially black holes (BHs). When two BHs -- that are expected to gather each a very dense cloud of DM -- start to dance, then the DM will likely over time carry away a substantial amount of energy, without telling us so -- it will still be dark. The DM gas heat will have to be carried away by other mechanisms instead.

At the very end, when the final inspiraling happens (that produces the GWs that we can see with LIGO/Virgo), the temperature of the DM gas must be absolutely crazy in the vicinity of the BHs, and thus it is expected to by utterly thin and not able to apply much breaking force. It will then instead be the GWs that carries away the final energy and the BHs can collide.

This is of course pure speculation, and I'm a computer scientist, not a physicist, but I hope someone (real astro-physicist or particle physicist) could do some initial calculations and write a paper about it.

//Gunnar