The image depicts a duo of clusters of galaxies as observed through the Chandra X-ray observatory. The red shading represents the ordinary gas that has become hot, while the blue shading represents the presence of dark matter, which is figured out by looking at the gravitational lensing effect.

So why is it? Because the cluster of galaxies is basically an ocean of dark matter, which is supposed to keep the galaxies and gas inside. However, this gas is sort of outside of the ocean of dark matter. After some studies, it turned out that this was the aftermath of a collision of two clusters at an incredible speed of 45000 km/s.

During the collision of two clusters, the gas within them indeed interacts, leading to heating and friction, which causes them to decelerate and remain behind due to gravitational pull. However, dark matter continues to move as though unaffected by the collision.