What's Dark Matter?

When scientists say that the majority of mass in the universe cannot be accounted for, which necessitates having the term dark matter, does this include an accurate estimate of all the dust and small particulate matter that space is full of?Makes me think of the biomass on earth and the majority of it being very small/microscopic though we normally only think of large animals which actually only make up a small percentage of the overall total.
- diobe (age 40)
Canada

Yes, that's all included. There are several ways of including all of those forms of familiar "baryonic"matter. One way is by comparing the densities of small nuclei (H, D, He³,He⁴, Li⁷). The nuclear processes that led to those ratios in the hot early universe are pretty well known, and lead to a unique value for the total density of that type of matter. Another way is by looking at ripples in the cosmic microwave background. The typical angular scale of some of those ripples also gives a value for the baryonic density, in agreement with the nuclear abundance value. (see .) There are other methods, also in agreement. The dark matter is something altogether else, something that interacts very weakly with baryonic matter and even with itself. You can see that because when galaxies pass through each other the baryonic matter is slowed down by the interactions but the dark matter goes through with little or no interruption.

Mike W.

(published on 12/24/2015)