Hot & Cold Dark Matter & WIMPs

Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

In the universe, components that cannot be seen are referred to as dark matter. Learn about hot and cold dark matter and explore what dark matter is made of, including MACHOs, WIMPs, and other particles. Updated: 10/27/2021

Dark Matter

What if I told you that what you see is really not what you get in the universe? If you look up at the night sky, all those shiny stars, they and others like them make up no more than 10% of all the matter in the universe. Essentially, you're missing out on at least 90% of matter in the universe. This matter does not emit nor absorb light. You can't see it with your eyes nor with a telescope. It is nonluminous matter; it is dark matter. While we can't see it, we can infer its presence through its gravitational influence on ordinary visible matter.

The different kinds of dark matter as well as what this mysterious dark matter might be made of will be explored in this lesson.

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  • 0:01 Dark Matter
  • 0:46 Hot & Cold Dark Matter
  • 2:14 What is Cold Dark…
  • 3:55 Lesson Summary
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Hot & Cold Dark Matter

The two forms of dark matter I'd like to discuss remind me of hot and cold drinks you could get at a local café or something. Hot dark matter is dark matter made of particles traveling at or near the speed of light. Cold dark matter refers to dark matter made of slowly moving particles.

This is easy to recall because you yourself can run a lot faster when it's hot outside and your muscles are all warmed up in comparison to cold weather, where you're going to run more slowly since your muscles are tense from the cold.

Additionally, the cold dark matter reminds me of a slushy you could get where chunks and clumps of ice stick to one another. The reason things can stick together in this cold drink is because the particles that make up the drink move slowly, lethargically, and can clump into pieces of ice as a result.

On the other hand, the hot dark matter reminds me of a hot drink, where the particles are moving really fast and therefore are part of very hot gases. These hot gases float away into the air around you and can't form any clumps of anything at all.

Since we need clumps of matter to stick together in a relatively small enough size to first form individual galaxies, astronomers believe most dark matter is thus made up of cold dark matter.

What is Cold Dark Matter Made Of?

At this point, it should be reasonable to ask: what is dark matter made of? Well, no one is fully certain. Some small amounts of dark matter might be made up of ordinary matter (that is to say, baryonic matter).

Therefore, one suggestion is that baryonic dark matter is made up of MACHOs, or massive (astrophysical) compact halo objects, such as black holes, white dwarfs, and brown dwarfs. Recent evidence suggests that this latter point isn't very valid, but I thought I'd fill you in anyways.

However, other matter, like that of cold and hot dark matter, might be made of sub-atomic particles that aren't ordinary matter (that is to say, nonbaryonic matter). Meaning, it's matter that's not made of protons and neutrons.

Hot dark matter may include neutral subatomic particles traveling at or near the speed of light called neutrinos.

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