Edwin Hubble discovered that the universe is expanding when he found that the further away a galaxy was, the more its spectral lines were redshifted. So nearby galaxies are moving away from us slowly, and far away galaxies are moving away much more quickly. Their apparent motion isn’t due to their motion through space, but the expansion of space itself.
The expanding universe can be compared to what happens when a loaf of bread with raisins in it bakes and expands in an oven. As the bread rises and expands, the raisins seem to move away from each other. They are not moving within the dough, but as the dough expands, the distance between them becomes greater. In this scenario, a raisin on one edge of the bread would see raisins on the opposite side of the bread moving away with a greater speed than nearby raisins.
Scientists describe the shape of the universe as flat, or very nearly so. Astronomers have determined that light travelling across the universe, even from extremely great distances, travels in straight lines. In other words, if we could point two identical lasers exactly parallel to each other in the same direction, the rays of light would remain parallel, never converging or diverging. The universe is three dimensional, but an analogy would be that if you drew two parallel lines on a flat sheet of paper, the lines would never converge or diverge. Another way to think of it is, a triangle drawn on a flat sheet of paper would have angles that sum to 180°. And if you started out walking in one direction on the sheet of paper, if you continued in a straight line, you would never return to your starting position.
If the universe were closed, that would be similar to drawing parallel lines on the surface of a sphere. They would converge. Or in the triangle example, the angles of a triangle drawn on the surface of a sphere would have a sum greater than 180°. And if you started out walking in a straight line on a sphere, you would eventually return to your starting point.
If the universe were open, that would be analogous to a saddle, where if you drew two parallel lines on it, they would diverge. The angles on a triangle drawn on an open shape like this would have a sum less than 180°. If you started walking in a straight line on this surface, you would never return to your starting position.
Scientists have been surprised by the results of experiments that show that the universe is flat, because the universe must have exactly the right amount of matter and energy to achieve this balance. However, there is not enough ordinary visible matter and energy in the universe to account for this and scientists have come to the conclusion that only 4% of the matter and energy in the universe is the visible type that we are used to. All the rest is dark matter and dark energy.
Dark matter is a mysterious form of matter that scientists believe must make up a large part of our universe. We cannot see it and we do not know what it is made of, but it contributes large amounts of mass to galaxies and galaxy clusters. In the 1960s Vera Rubin studied the rotation of galaxies and found that their rotational velocity could not be explained by the mass of the visible matter in the galaxy.
Several theories were proposed. One was that another form of matter, dark matter, is present in galaxies and contributes greatly to their mass.
There are several other theories, usually called MOND for Modified Newtonian Dynamics, which state that Newtonian mechanics is not well understood at such large scales and that perhaps gravity behaves differently on the scale of a galaxy or galaxy cluster. The various MOND theories have different ways of describing how Newtonian mechanics, particularly gravity, might be altered to better model what scientists observe. Most scientists believe that there is more evidence supporting the theory of dark matter than MOND.
The universe is flat, or nearly so, but does not have enough visible matter or dark matter to account for this. To make up for this, scientists theorize that there must be another form of energy in the universe that we can’t detect. This mysterious “dark energy” accounts for about 70% of the contents of the universe!
On very large scales, the universe has a filamentary structure with galaxies and galaxy clusters arranged in super clusters and sheets. Some scientists describe the structure as being similar to soap bubbles. These sheets trace a web-like pattern and are called the cosmic web. Dark matter clumps together in between the filaments in the cosmic web.