Agreed. But to jump from that statement to, "There must be some magical invisible undetectable fairy dust substance for which we have to take the presence of based on faith that we will call Dark Matter," isn't science anymore. I am in the camp that such exotic explanations are likely unnecessary. Far more likely, there is normal matter/mass in the universe that we're simply not seeing because it is not currently involved in a massive ongoing nuclear fireball (star).
When calculating the mass of the universe, there are several known bits.
We know the mass of stars.
We know the locations of and can get a pretty solid estimate of the mass of black holes
We know the -hot- portion of the IGM (Intergalactic Medium) - the part that emits X-rays and other radiation and can estimate the net mass/energy of that.
We know from observation that there must be more mass/energy in the universe than the above can account for.
Jumping from, "We know that there is mass unaccounted for" to "Exotic Fairy Dust (Dark Matter / Dark Energy) that we cannot see or detect is 95% of the mass/energy of the universe" is a cop out.
Much more likely, we have a higher density of high velocity neutrons and hydrogen in the Intergalactic Medium than previous guesstimates can account for.
The volume of the universe is estimated at 3.58*10^80 m^3.
The mass of observable 'ordinary matter', which translates to 'hot observable matter' is around 1.5*10^54 kg.
The 'missing mass/energy' that gets called "Dark Matter" is estimated at around 20 times the 'ordinary matter' - which would be around 3*10^55 kg.
The mass of a neutron is 1.67492749805×10−27 kg.
If the entirety of "Dark Matter" were replaced with neutrons, that would be 8.95*10^80 neutrons.
Spread those 8.95*10^80 neutrons evenly through the 3.58*10^80 m^3 of volume results in an average density of 2.5 -static- individual neutrons per cubic meter of space. The faster those neutrons are moving, the fewer of them would be needed to make the calculation work (energy).
So, which hypothesis is more likely to be true?
A. That we haven't got detection equipment necessary to spot an average additional density of 2.5 "cold" non-radiating neutrons per cubic meter of intergalactic space.
or
B. That there is this exotic fairy dust that we cannot see, classify or detect called "Dark Matter" that is the equivalent of 95% of the mass/energy of the universe.
