To better understand our universe, it is often necessary to estimate the mass of an astrophysical object. Those objects can be found across a vast range, from the size of the Sun, the solar system, the Milky Way, and even the entire universe.
Researchers use a number of techniques to measure the mass of extremely large objects. One way to estimate an object’s mass is by observing its light output. If the object does not emit its own light, researchers can examine the way in which the light of background sources bends around it. Another technique is to examine the dynamic motion of objects around it.
It was long believed that the estimated masses coming from these techniques would agree with one another. However, over the past 80 years, it has become apparent that for objects at the galaxy scale and larger, the amount of mass contained exceeds the mass of its luminous constituents. This additional mass, which cannot be accounted for by luminous matter we know about, has been coined “dark matter.”
From our understanding of how matter was created in the early universe, we now believe that dark matter is composed of fundamentally new particles
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The brightness of a galaxy can be used to estimate it’s stellar mass. Using this mass estimate and Newton’s laws, we can predict how fast stars and dust should travel around the outside of the galaxy. However, when we measure the velocity of the stars in galaxies like M33 (image below), we find that they travel much faster than expected. This presents one of strongest pieces of evidence that galaxies sit in much larger halos of dark matter.
The image shows the collision of two massive clusters of galaxies. The total mass of the cluster can be mapped by the gravitational bending of light (blue), while the bulk of the luminous matter resides in the form of hot, x-ray emitting gas (pink). When these clusters collided, the hot gas was slowed by drag interactions, while the dark matter clumps pass through each other without interacting.