>>How do you explaining gravitational lensing with no visible mass, without dark matter? >Black holes?

Well, black holes are not really black, you can see their accretion disk. Also, a centralized huge mass would create a different velocity profile from what we are seeing. So black holes are pretty much out.

Only if they have an acretion disk. If they've cleared their neighborhood of most infalling matter then they would be invisible except by gravitational effects.

I am not certain about the mechanics of it, but one of the most prominent theories right now is that dark matter is actually primordial black holes 1.

1. https://www.nasa.gov/feature/goddard/2016/nasa-scientist-sug...

Sorry, but I don't understand your argument. When asked 'how do you explain lensing without dark matter?', your answer is 'with this particular flavor of dark matter'?

Well we're seeing a bunch of stuff happening out there and calling it dark matter.

1. The big problem is not enough stuff. Galaxies don't have enough stuff in them to let gravity hold them together.

2. A second problem is that we have stuff out there that our instruments can't detect but seems to be putting out gravity and bending the light.

The orthodoxy of dark matter says 1 and 2 are connected. That dark matter is bending light and causing extra stuff in galaxies to allow them to form and keep them from flying apart.

This physicist says no, just gravity doesn't work quite the way we think it does, because the stars in galaxy are moving at a constant speed relative to their distance from the galaxies center - but if there were a whole bunch of extra stuff in there that we can't see creating extra gravity all over the place, it should mess with the speed and cause it to be different depending on what parts had more dark matter in it.

My parent was saying, how do you explain the invisible stuff putting out gravity and bending light, and suggested that it could not be explained by black holes. I was pointing out that one of the main explanations for it is black holes, but that still doesn't solve the first big problem of not enough stuff.

I see now, makes sense.

> Also, a centralized huge mass would create a different velocity profile from what we are seeing.

Doesn't need to be centralized, no? They can orbit the galaxy as the other bodies.

Well, right. You could have a distribution of black holes. But that would mean a lot of black holes - all of them in space so void of other matter that they have no accretion disks. Also, with black holes so common, we would see many collisions with plain stars - and those would be very visible.

Can't be black holes. For one thing they would start becoming visible through Hawking radiation if they were small enough to be homogenous like dark matter. Secondly baryon acoustic oscillations set a limit on how many black holes there can be.

For your second part, the constant velocity is very well explained. The dark matter is just very broadly distributed. That's a natural consequence of not interacting with anything. It clumps up much less.

How does this dark matter have enough gravitational effect to change the structure of the universe, yet not clump together? It doesn't make sense.

Unless it doesn't exhibit gravitational properties with itself?

Just a thought; I have no training or experience here.