In simple terms, why are scientists so convinced that most of the universe is made up of dark matter and dark energy?
When I first heard this, it seemed as if the data were being fudged. Since the data do not agree with the theory, we will just have to create new data.
I can understand that observations require more matter and energy than we can account for, but why must it be something completely different from ordinary matter and energy? What makes us so sure that we can account for all of the ordinary matter and energy? And what does it mean when they say that dark energy makes up 72% of the universe? How do you compare matter and energy? The only way that I know of would be to apply the formula E=mc^2 to matter. That would be an awful lot of energy.
Where is this dark matter and energy supposed to be lurking? Is there any of it in our galaxy, in our solar system? It all seems really bizarre and in need of an explanation understandable by the common person.
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Yeah, it’s confusing stuff.
Dark matter is present practically everywhere, we know it is there because something with force is acting upon things, we can prove this, and the only logical explanation is that it is dark matter. The whole thing is confusing because technically it is theoretical, because we can not prove it absolutely, but something has to be there. If there was not, the things that have been affected would have acted entirely differently.
As far as dark energy compared to regular energy, its basically just what we can measure, and what we can’t.
From the top of my hat:
We know how many stars there are in our galaxy, we know how much cumulative mass they have, and we know the size of our galaxy and the rate at which it spins.
Basically, there is not enough visible mass in the galaxy (and by extension the universe) to hold the galaxy together in its current cluster, even considering the supermassive black hole at the centre, so logically, there must be more mass in the galaxy than is currently visible.
Then there is the fact that dark matter has an observable effect on light (gravitational lensing).
So we “know” dark matter exists because of its observed effect on the universe and because the observed universe could not work without it based on what we do know about the universe.
To sum it all up:
Dark matter: Rotational speeds of galaxies would otherwise drive stars apart (centrifugal force), despite the existence of supermassive black holes in their centers
Dark energy: Type Ia supernovae act as standard candles in distant galaxies (we can calculate the distances) and they show that the universe isn’t just expanding – the expansion is accelerating – energy is required to counter gravitational pull of galaxy clusters
They are very confident that something exists and exerts gravity, because we have seen the effects of objects being pulled by the gravity. There are competing ideas as to where the extra gravity comes from, including large numbers of unknown particles, or gravity from other universes. Some have proposed that the known laws of gravity may be incorrect.
Yes, as @PhiNotPi notes, you can infer accurately that something exists when you are only able to observe the effect it produces. For instance, we cannot see the supermassive black hole at the center of our galaxy, or any other black hole for that matter. But we can clearly observe the effects produced by black holes.
Dark matter, or something that produces gravity, must exist to keep our local galactic group and others like it from flying apart given their rotational momentum. Dark energy must exist to produce the observed accelerating expansion of open space between local galactic groups.
I read that scientists began with the theory that nothing does not exist, so where we see nothing, there must be something there.
We cannot use deductive logic to prove the existence of dark matter. We must, and do, use inductive logic to necessarily induce an unseen agent as a specific cause to a specific effect.
It’s kind of like looking out your window and seeing the tree branches moving around. You cannot feel or detect the wind from your position. So you must induce the idea of wind as a necessary cause which produces the effect of moving tree branches. Or you could induce the idea that tree branches have muscles and are moving on their own.
There are discrepancies between the mass of astronomical objects as calculated from their gravitational effects and the mass of luminous matter they contain in the form of stars, gas and dust. For example stars in the Milky Way orbit the galaxy faster than the calculations say they should, even allowing for the mass of our galaxy’s black hole.
What causes this discrepancy is a mystery but it is thought to be an undiscovered type of subatomic particle.
Another inexplicable fact is that the rate of expansion of the universe is not decreasing due to gravity as you would expect but is increasing. Something is causing the universe to expand faster and faster and we have no idea what it is but we call it dark energy.
The dark matter lurks where there is visible matter as the two types of matter attract each other gravitationally. There have been attempts to create maps of dark matter by studying these gravitational effects.
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