Question

Why is the luminous matter so much more compact than the dark matter?

Answer 1

For our consideration let us say that luminous matter interacts with itself in two ways, gravitationally and electromagnetically. Where as dark matter interacts noticebally with itself only gravitationally. So, when the luminous matter interats with itself gravitationally it starts to attract and when they are close enough they undergo random collisions. This collisions causes energy losses mostly in form of heat and light. Since the matter has now lost a bit of energy through its electromagnetic interaction, the next time when gravitational effects are experienced, the matter has lesser energy and the particles come much closer, so eventually, this cycle causes the luminous matter to get more and more compact with each interaction, eventually forming everything we know in the observable universe, stars, planets , dust clouds, etc. Where as when the dark matter interact it's purely gravitational, when the matter is attracted due to gravity the matter comes close together, but as they are gettting close they accelerate and since they don't interact electromagnetically, they don't collide and they just pass through, and the gravitational is also weak, so it can't hold the particles. These matter which passes through will again be attracted back by gravity once gravitational force is strong enough, and this causes an oscillation where the matter just pass through. Thus they can't become compact.