How do we calculate the mass of the universe taking into consideration dark matter? Mass of visible matter can be computed by stellar method, but how do we calculate mass of dark matter which we don't see? Why do we assume that dark matter is more abundant than visible matter?
Check out the Wikipedia page on the missing baryon problem. It's how we compute the mass of ordinary matter. You can see there are methods of getting the mass of ordinary matter without adding them up. In other words, you can find ordinary matter even if they're invisible.
This is similar to how we find dark matter. Again, look at Wikipedia's article on dark matter, in particular the observational evidence section. Ignore the more technical methods if you're new to the field and just look at the first few:
- With galaxy rotation curves, you know you need more matter to recover Kepler's laws. How much matter? You can tweak that number and therefore get an estimate for the dark matter needed. You can't prove that this dark matter isn't just invisible ordinary matter using this method, however.
- The same goes for velocity dispersions.
- Gravitational lensing can measure the total mass of an intervening cluster without knowing anything about the kinematics of the cluster.
If you're wondering how we know dark matter can't just be invisible ordinary matter, again look at the Wikipedia article, in particular this section.
- Big bang nucleosynthesis constrains the amount of ordinary matter in the universe. If dark matter were just invisible ordinary matter, we also expect more helium, lithium, etc in the universe than we observe.
- Gravitational microlensing constrains the amount of MACHOs (massive compact halo objects, such as black holes and invisible planets) we can have in a large mass range, covering most of the plausible candidates.
- Data from the cosmic microwave background indicates a large fraction of matter does not react with photons.