What exactly is dark matter? How about an evaporated black hole?
A radius of a black hole is typically 2Gm/c^2 or less. (G=Newton’s constant; m=black hole mass; c=light speed.) When the radius (r) becomes Gm/c^2, the black hole’s energy is 100% gravity; as the radius shrinks, so does the mass, and the mass density increases, so does the gravity. Theoretically, the black hole could reduce to a point particle.
But here’s the question: When it becomes a point particle, is it something or nothing? If it’s something, gravity is infinite. If it’s nothing, gravity is zero. We won’t know which it is until we measure it.
Now imagine a space filled with a mixture of particles that are basically dead or dying black holes. Some might have infinite gravity; some might have zero gravity; some might have finite gravity. If we calculate the average of all that gravity we get a finite amount of gravity that corresponds to nearly empty space. What little matter there is does not emit radiation that we can detect, since this matter is 100% gravity. It’s strong, weak and EM forces were spent long ago during the evaporation process.
It is not certain that dark matter exists, but there seems to be more gravity than the current inventory of matter can explain. Here's the mathematics pertaining to dark matter:
If we start with Einstein's and Newton's equations, we can derive 3 and 4--the amount of energy devoted to gravity out of a total energy of mc^2.
Ordinary matter is represented at 3. Only a small part of its total energy is devoted to gravity. the majority of its energy corresponds with the electromagnetic (EM), weak and strong forces. We can detect ordinary matter. It emits EM radiation (light, radio waves, etc.).
Dark matter, on the other hand, has virtually all its energy devoted to gravity (see 4). It emits no EM radiation so it is undetectable.
The big bang singularity was possibly also highly concentrated matter with all its energy devoted to gravity. The other forces evolved when the universe expanded. The greater the radius of the matter and space-time, the less mass density there is, and so gravity becomes weaker and the other forces emerge.
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