The Matter of Whether it’s Hot or Cold Dark Matter.
Since dark matter does not emit electromagnetic energy that we can detect (hence the name dark matter) its existence is surmised by the gravitational influence it has on baryonic matter - protons and neutrons, the matter of the stuff around us. For example:
- Galaxies rotate faster than they should, given the amount of mass we can detect in them.
- Large galaxy surveys reveal that galaxies exist in superclusters, forming walls, filaments and bubbles stretching across vast regions of the cosmos.
We see gravitational lensing by galaxy clusters that cannot be attributed to the mass of the visible material we observe.
Hot dark matter can, to a degree, account for these super-large structures, but cannot account for the formation of matter on a smaller scale, like galaxies. Neutrinos are a likely candidate particle for hot dark matter, because they have no mass, or a very small amount of rest mass. But, the neutrino is not massive enough, and at the time of the big bang, their high speed and high density—being the dominant particle of matter at that time—would have smoothed out matter fluctuations that would have led to the creation of galaxies as we see them today. This is called the “Top Down" model of the universe, where the smooth distribution of matter broke apart to form the objects we see today.
Cold dark matter provides a solution that appears to be a better fit to what we observe today, but it is not without its problems. The theoretical WIMP, or Weakly Interactive Massive Particle, that makes up CDM has yet to be discovered, but computer models based on CDM tend to display structures like we see in the cosmos. CDM is associated with the “Bottom Up" model of the universe, where small clumps of matter aggregated into the galaxies and clusters of galaxies we observe with our deep sky images and especially in the 2MASS and Hubble's COSMOS surveys.