Hot dark matter

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Hot dark matter

What is it
Hot dark matter (HDM) is a theoretical form of dark matter made of particles that travel at ultrarelativistic speeds (very close to the speed of light). Dark matter is matter that does not emit or absorb light, so we can’t see it directly, but we can detect its gravity through how it shapes galaxies and the universe.

HDM vs other dark matter
Dark matter can be imagined as having different speeds, which come from the mass of the particles:
- Hot dark matter: lighter particles that move very fast.
- Cold dark matter: heavier particles that move slowly.
- Warm dark matter: in between.

Role in galaxy formation
If most dark matter were hot, large cosmic structures would form first, surrounding big clusters, and the formation of small galaxies would be harder to explain. However, measurements of the cosmic microwave background show the early universe was very uniform, and ultrafast HDM would not clump into small galaxies quickly enough. This means hot dark matter alone cannot explain all of cosmic structure. Scientists often consider mixed dark matter (a combination of hot and cold) or mainly cold dark matter to explain how galaxies and clusters form.

Neutrinos as an example
Neutrinos are an example of hot dark matter. They have very small masses and interact only weakly (and gravity) with other matter, making them very hard to detect. Experiments like Super-Kamiokande study these elusive particles.

Bottom line
Hot dark matter helped scientists think about how structures in the universe form, but the current view favors cold or mixed dark matter as the main way the universe builds galaxies and clusters.