1.2 The Case for Dark Matter

summary

Current Understanding of Dark Matter

Dark matter does not interact with any electromagnetic radiation–it does not emit, absorb, or reflect light.
Dark matter moves very slowly compared to the speed of it. In other words, dark matter is cold.
It makes up almost one quarter (25%) of the total mass-energy budget of the universe. That is six times more than ordinary (or baryonic) matter (which makes up about 5%). Dark energy makes up the remaining ~70%.

The Dark Matter Halo

In a solar system, most of the mass is concentrated at the center in the system’s star.
In systems with most of the mass concentrated at the center, objects that are closer to the center orbit faster than those on the edges. The velocity of planets should monotonically decrease with increasing distance from the star.
Galaxies generally have masses between 108 and 1012 the mass of the sun. This size varies significantly from galaxy to galaxy. Much like in a solar system, most of a galaxy’s mass is concentrated at its center.
This would lead to a predicted velocity curve for galaxies that is similar to that for solar systems – orbital speed would decrease with distance from the center.
Instead, what astronomers have found is that objects at the edges of galaxies move significantly more rapidly than predicted using modern physics.
This increase in velocity at the edges of galaxies can be explained by a dark matter halo. This is a proposed ring of dark matter that adds mass (and therefore increased orbital velocity) at the edges of galaxies.

Galaxy Clusters

Galaxy clusters are the most massive bound objects in the universe. The most massive clusters have masses on the order of 1014 to 1015 solar masses, and can be up to 3 million light-years across.
The name galaxy cluster may, however, be a misnomer. Galaxies themselves only compose about 1% of the mass of galaxy clusters. Clusters also contain large quantities of high-energy gas that contribute about 10% of the cluster mass. Dark matter is thought to make up the remaining ~90% of cluster mass.
One piece of evidence for this huge contribution of dark matter is, like within galaxies themselves, that objects in clusters move much faster than they would if there was no dark matter present. This is also evident in the radiated light from the high-energy gases, whose energy would be explained by the presence of dark matter.

Gravitational Lensing and the Bullet Cluster

Einstein taught us that everything is influenced by the presence of gravity, including light. This is observable as the phenomenon of gravitational lensing, which is when light bends around massive objects like galaxies. This can cause the same object to be visually “duplicated” in the sky.
This lensing would not be possible if galaxies did contain large quantities of dark matter whose enormous mass acts as a gravitational lens on passing light.
The bullet cluster provides another piece of evidence in favor of dark matter. The cluster is the aftermath of the collision of two once-separate clusters.
When two clusters collide, their high-energy gases will be slowed down by their interactions with each other. Dark matter, however, reacts so weakly that it essentially can only interact through gravity. This means that the dark matter clouds of each galaxy will pass by each other without slowing, overshooting the gas clouds.
This is evidenced by the gravitational lensing around the bullet cluster. Light is not bent around the gas cloud, which contains most of the baryonic matter in the bullet cluster. Instead it bends around a whole different volume of space that contains the galaxies, which are bound to the dark matter.

The Cosmic Web

Simulations that allow dark matter to evolve from a homogenous initial state have found that over time the dark matter forms a network of clumps connected by filaments.
These simulations fit in perfectly with the actual observed large-scale structure of matter in the universe. The clumps of dark matter align with clumps of galaxies.