The Standard Model describes all of the matter that we know. However, if you study the universe around us carefully, you find that the matter described in the Standard Model only makes up roughly 4% of the universe. The missing mysterious substances have been dubbed dark matter and dark energy.
These mysterious substances cannot be seen directly, hence the name dark, however you can infer their existence by their gravitational effect on matter that you can see. A lot of astronomical discoveries were made by observing closely the objects that have already been found. For example, in 1846, a Frenchman called Urbain Le Verrier, studied the motion of Uranus very carfully and concluded that its path could not be explained by the known gravitational forces acting on it. He wrote a letter, explaining his observations, to the director of the Berlin Obsevatory, Johann Gottried Galle, who, following Le Verrier’s directions, discovered Neptune. This tactic is still used in modern cosmology, but instead of discovering new planets, astronomers have discovered dark matter. Astronomers first inferred its existence in the 1930s. They found that galaxies are surrounded by rings of dark matter, which scarcely interact with normal matter or even with itself. All it seems to do is to interact via gravity, providing scaffolding on which the luminous matter of the galaxies rest. The even more abundant dark energy, on the other hand, appears to accelerate the expansion of the cosmos.
The most persuasive evidence for dark matter to date is from observations of the Bullet Cluster(above). The cluster is actually made up of two galaxies that have collided. The stars of the galaxies did not collide because they are extremely small compared to the galaxy. However, their gas clouds have collided emitting x-rays, shown in pink on the picture. They measured the total mass of the cluster with the use of a phenomenon called gravitational lensing. This occurs when the cluster’s gravity distorts the light coming from background galaxies, the greater the distortion the greater the cluster’s gravity, hence a heavier mass. They then compared these measurements with the x-ray images of the luminous matter in the cluster, in order to find out how much of the clusters matter is made up of normal matter which we are used to. This resulted from a large amount of mass being unaccounted for, therefore allowing them to map the location and density of the dark matter. The results confirmed dark matter’s inertness, the clumps of dark matter from the galaxies sailed past each other as if there was nothing in their way. While, on the other hand, the luminous matter collided spectacularly.
This clearly shows that this unknown dark matter is something not yet fully understood and hence not included in the Standard Model. There are many theories as to what dark matter could be, including WIMPs which stands for weakly interacting massive particle. But whatever dark matter is, it is not included in the Standard Model; therefore the Standard Model must be incomplete.