Dark Matter

[samsara15]

Why is there a need for Dark Matte'r? I understand that 'Dark Energy' fules the expansion of the universes, but what makes it necessary for there to be 'Dark Matter'?

[ToSeek]

I think the two main reasons are:

• - To explain galactic rotation rates
  
  - To provide sufficient gravitational attraction to form the large scale structures in the universe in the time since the Big Bang.

Here's a good tutorial.

[Spaceman Spiff]

I think the two main reasons are:

• - To explain galactic rotation rates
  
  - To provide sufficient gravitational attraction to form the large scale structures in the universe in the time since the Big Bang.

Here's a good tutorial.

Add to that:
-motions of galaxies within galaxy clusters
-the presence of gobs of high temperature gas in clusters of galaxies
-the magnitude of gravitational lensing by foreground galaxy clusters of distant galaxies (go here and here)

Here is another link of interest.