Dark Matter?

Question

please explain in the simplest terms.
i have read many websites, but i dont fully grasp the concept.

please please please do not copy and paste a website's words

if you don't know, don't answer

ok so why cant it be dark energy holding galaxies together?

Answer 1

Well basically when you look at a galaxy that is spinning around its center of mass, the rate at which it is spinning is too fast for it to remain cohesive. If all a galaxy was composed of was the matter we could see, it would be too light and would simply hurl itself into space. The only way a galaxy can have enough mass to remain a galaxy is if there is a significant amount of matter we haven't been able to detect. This undetected matter is "dark matter". Quite literally, it's matter we haven't seen. That doesn't mean there aren't other ways to detect it though. Observing the effects of gravity is one way to observe matter we can't directly see.

Answer 2

look at it this way. the sun is a beach ball. walk about 1 and half miles away and put poppy seed on the ground. the poppy seed is pluto. the beach ball the sun. the speed which the poppy seed is moving is fast enough to stay in a stable orbit around that sun 1 1/2 miles away. now walk 10 miles away drop a bunch of poppy seeds that move at the same speed. (the should fly off given normal models. but for some reason the stay in a ginormous orbit? Is it the mutual gravity of this cloud of poppy seeds making it a bigger object mass? hence mutual collective gravity? no can't be that. so there must be unseen bits in there to help the gravity of these very distant poppy seeds stay in orbit somehow. and they must be 20x more dominant then the poppy seeds themselves! thats the problem. the beach ball now instead of being a model for our sun is now the model of the core of the galaxy. and the cloud of poppy seeds is the spiral arms of a galaxy. they are moving around an object but the center object cannot have the gravity to keep the arms moving at that speed in that orbit. the gravity 'dimple' is not that strong way out there. so something must be doing it. if you make the spiral arms MUCH more dense with dark matter it solves the problem nicely. but what is this invisible undetectable dark stuff? Another theory is M.O.N.D. or Modified Newtonian dynamics. its where at vast distances and vast objects the laws of gravity are not equal in size and distance as to smaller objects (sun/planets VS supermassive blackhole/star clouds)

Answer 3

P.I. Joe gave a good answer, but in your follow up you asked about "dark energy." Don't confuse that with "dark matter;" the two are entirely different. So-called dark energy is the unknown, mysterious force that is causing the expansion rate of the universe to accelerate instead of slow down as would be expected.

Answer 4

The absence of all matter
Like my ex.
Just an absence.