Bullet Cluster: A Compelling Proof of Dark Matter

Image description: Composite image of the Bullet Cluster showing the separation between visible baryonic matter (in pink) and non-baryonic dark matter (in blue). Image source: Chandra X-Ray Observatory.

Bullet Cluster: Collision of Two Galaxy Clusters

The Bullet Cluster is a galaxy cluster located about 3.8 billion light-years from Earth. It is famous for providing one of the most compelling pieces of evidence for the existence of dark matter. This cluster is the result of a collision between two galaxy clusters, an event that allowed scientists to directly observe the effects of dark matter.

The collision between the two galaxy clusters occurred about 150 million years ago. This collision separated the visible baryonic matter (in pink) from the non-baryonic dark matter (in blue), allowing scientists to study them separately.

When Dark Matter Reveals Itself

• Separation of Hot Gas: The hot gas, baryonic matter (in pink), observable in X-rays, was slowed down by frictional forces during the collision, forming a bullet-like structure (hence the name "Bullet Cluster").
• Distribution of Dark Matter: Dark matter, non-baryonic matter (in blue), does not significantly interact with gas or light, so it was not slowed down, allowing it to be located through the gravitational lensing effect.
• Gravitational Lensing Effect: Dark matter, much more massive than stars, distorts the light of objects behind it, allowing its distribution to be mapped.

An Undeniable Proof of Dark Matter

The Bullet Cluster provides three major pieces of evidence for the existence of dark matter:

• Spatial Separation: Dark matter and hot gas are spatially separated, indicating that dark matter does not interact in the same way as visible matter.
• Missing Mass: The total mass of the cluster, calculated through the gravitational lensing effect, is much greater than the visible mass, suggesting the presence of invisible mass (dark matter).
• Dynamic Behavior: Dark matter follows a different trajectory from the hot gas, confirming its independent existence.

Position of Stars in the Bullet Cluster

A galaxy is primarily composed of dark matter (about 85%), followed by stars (10-15%), and then gas and dust (1-10%).

The stars of the galaxies in the two colliding clusters are mainly located near the regions where the dark matter (in blue) is found. Stars, like dark matter, do not interact strongly through friction or collision.

Since galaxies are primarily composed of empty space, the stars pass through the collision zone without being significantly slowed down, unlike the hot gas.