No one answered, so perhaps I was not clear. There is a bug in the options menu. If you try to correct the problem in the options, the density will lock, even if you unlock it. So, the bug it is in the density, probably.

I will explain the physics of it and compare with the simulation, so that things will be clear.

A black hole has an escape velocity of 1 light speed at its horizons. If I get 2 black holes of equal mass and collide them, you will get twice the mass, but in the simulation the escape velocity will become sqr(2), which the game gives as 1.41.

The correct result in physics should be twice the mass, escape velocity still the same, and the radius become twice as big, which means that the density falls by 1/8.

I will give some other practical examples. A sun sized black hole has a radius of ~3 km. A 10 million sun mass black hole will have 30 million km, that is 3*10 million. A 10 thousand sun mass black hole will have 3*10 000= 30,000 km of radius. You can check all that on universe sandbox, by creating black holes from the menu. So, if I collide 2 black holes of 10 million masses, I will get a black hole with 20 million masses, 2 the radius, that is 60 million kms, and 1/8 the density.

In the program, though, there is a wrong output. If you collide 2 black holes, the radius will remain the same, the density will double, thus you will have a speed of light which is sqr(2). That is, a black hole is treated like a common object where the density only doubled. If you collide 10 million masses black holes, you will get a 20 million mass black hole, but with the same radius, and 2x the mass.