Malkin6701 said:
... What is dark matter/dark energy? What is the difference between the two? ... I was wondering was if it is a solid, liquid, or gas? Or maybe something completely different? If dark matter makes up most of the universe... Why can't we find any? ...
The first thing to realize is that you may not recognise everything that could be called ‘mass’ or ‘energy’. For example, an electric field has mass. The second thing to realize is that the terms mass and energy can be misunderstood, because they change their nature depending on the observer. For example, an asteroid has rest mass which is the mass someone standing on the asteroid would measure, but if we think of it as moving the mass is equal to the rest mass plus the kinetic energy. A photon has no rest mass, the mass of a photon is the mass of its kinetic energy.
When we talk about objects in cosmology the description of mass becomes a bit more complicated again. We need to talk about the density of the matter, so a moving cloud of dust (say) gets more massive because it is moving, but it also gets more dense because length contraction means it takes up less space.
There are other complications which I will glide over by assuming that we want to describe a point in space in a way that someone at that point would consider ‘natural’, meaning that we talk about 3 space dimensions, mutually orthogonal (ie at right angles), and an orthogonal (ie unbiased) time dimension.
Given those assumptions, the point in space will have an Einstein tensor value that can be thought of as representing 10 Newtonian values. One will be the mass density, 3 will be the momentum density in each of the three directions in space, 3 will be the linear pressure (or tension) in each of the 3 directions, and the last 3 will be the twisting stress in each of the 3 directions.
These values depend on how you view the point, but given all 10 values for one viewpoint, we could calculate what the values would be given some other viewpoint.
When talking about space expanding or contracting, we are talking about a property of a large volume of space time: a small volume can always be described as expanding, contracting, or staying the same. In choosing a ‘natural’ way of describing space, I have in effect chosen to describe the small volume of interest as staying the same. However, it only stays the same for an instant, after which it might start to collapse or expand. The collapse we would ascribe to matter, both dark and non-dark. The expansion we would ascribe to dark energy. In fact, both may play a part, with one partly or wholly masking the other.
So now I am able to say what is matter, and what is dark energy. Matter (in this sense) is mass, rest plus kinetic energy, plus pressure, dark energy is tension plus negative mass. In the early days it was assumed that the pressure or tension would be negligible and that negative mass could not exist, and the only thing that had a significant effect in our era is the mass. Einstein introduced a ‘cosmological constant’ which is a mixture of tension and mass (or of negative mass and pressure) when he thought he needed it to balance the tendency of the universe to expand or collapse. The mixture has the special property that makes it convenient to work with, and when you are guessing about something, you might as well guess something that it is convenient!
Once the theory of the big bang origins became accepted, the cosmological constant became ignored. However, we again find that we can’t explain the rates of expansion and contraction of space. Galaxies generally spin, and gravity stops them from flying apart, but there does not seem to be enough mass in the galaxies to hold them together, so we guess there is a lot more mass there that we don’t know about, and we call that dark matter. On the other hand, the universe as a whole seems to be accelerating apart, so we guess that between the galaxies there is a lot of dark energy.
Now as to why we can’t find any. First, although we think there is a lot of dark matter and dark energy in the universe, it is spread very thinly, unlike ordinary matter which clumps together to form stars and planets. Second, we have not found anything that can couple with the dark energy or dark matter. By couple, I mean exert a force on it. Third (really expanding on the second point) the dark energy and dark matter do not seem to absorb or emit light. The only effects we have been able to observe is the effect of its gravity (positive: dark matter, negative: dark energy) and gravitational lensing (for dark matter, in some special cases).
