I figured it would be some absurdly long time scale. I didn't know about the timescale of earth losing energy to gravitational waves & crashing into the sun though; thanks :)

We can observe these black holes, if they existed, via their lensing effect on light passing near it

If you watch the night sky very carefully (with telescopes of course) and train it on a faraway galaxy, you will be able to observe the mass of anything that passes between you and that galaxy. Any massive object would bend the light eeeever so slightly, and you would see the galaxy wiggle in the background

Extremely small black holes would need to exist in such density that they’d need to permeate everywhere. Excuse for a moment that they’d need to be passing through our solar system regularly (this is fine, but we’d detect them no problem), their effects on viewing distant objects in the cosmos would be unmistakable

Also, we can rule a HUGE range of masses for black holes because anything less massive than ~200 million metric tons would’ve evaporated by now. Black holes of this mass would be the trickiest to spot, but fortunately, Hawking has proven they literally can’t exist if they formed in the big bang

edit: another problem with small black holes is that they’d have charge and they’d rotate, both of which make them MUCH “clumpier”. We’d end up with swathes of intermediate and supermassive black holes (MANY more than what we currently see), and these would be trivial to observe

I suppose that, technically, gravity isn’t conservative. Energy is lose to gravitational waves as WIMPs pass one another and they eventually COULD collapse into black holes, given sufficient (spitballing number here, no math done) quintillions of years

Signs are pointing to this being a very unlikely candidate for dark matter

They’re a lot of things Cold Dark Matter theories can explain than stellar mass black hole models can’t

CDM explains baryonic acoustic oscillations, why some galaxies are missing DM while most have a halo of it, the evolution of the structure of the universe, etc

This discounts the mounting evidence against swarms of stellar mass black holes. Dark matter outweighs luminous matter approximately 6 to 1. That means for every star in the sky, there should be a black hole ~6x as massive (on average). That’s a lot of huge black holes!!

We’ve surveyed the night sky for thousands of hours, looking for any signs of stellar mass gravitational lensing, but we just don’t find it in sufficient quantity that could suggest any number of any size black hole can even come close to the mass required to be dark matter

tl;dr: there’s a lot of evidence that suggests DM isn’t black holes