Image 1 Caption: A pie chart showing the calculated amounts of mass-energy in the observable universe. Beyond the observable universe, we have speculative theories, but which if any are correct, we do NOT know.

Features:

1. The two concepts mass and energy developed independently. The advent of theory of relativity showed they were different manifestations of the same thing which in relativity speak is mass-energy. The unifying result is the mass-energy equivalence which as a formula is E=mc**2---the only physics formula everyone knows.

   Actually, people often now use mass, energy, and mass-energy as synonyms. They use whatever seems suitable for their use. Mass-energy tends to be used when the mass-energy equivalence is being emphasized---or they are just being verbose. Note:

   1. Mass tends to be used when speaking of the total amount of mass-energy in a place or the total amount per unit volume in one place because it is the total amount that goes into calculating the gravitational field or gravitational force (i.e., mass as gravitational mass) and the resistance to acceleration (i.e., mass as inertial mass). In classical physics, that the two kinds of mass are the same thing is an unexplained coincidence. In general relativity (GR), it is axiom called the equivalence principle.
   2. Energy tends to be used when speaking of various forms energy: multiple forms of which can be in one place. Each form has its own special properties: importantly, its own formula for calculating its value and its own force law. However, all energy is energy since any form can be converted into any other form with energy conservation holding. The conversions are NOT necessarily easy. Note in general relativity, the simple principle of energy conservation does NOT actually hold, but a generalization of it does.

2. It is the total mass-energy that determines cosmic evolution (i.e., expansion of the universe) and the large-scale structure of the universe (and the cosmic web which is much the same thing).

3. The rather precise dark energy and dark matter values in the pie chart are based on the Λ-CDM model which is the considered the standard model of cosmology (SMC, Λ-CDM model) circa 1995 -- present (Scott (2018)) and is fitted to many observations. However, the Λ-CDM model may need substantial revision or replacement in the 2020s in which case those values might change significantly.

4. Note all the values in the pie chart have some uncertainties even give the Λ-CDM model as absolutely true (which it may NOT be). Other references will give slightly different sets of values based on different analyses, but all sets will probably agree or nearly to within uncertainties of a few percent.

   

5. Image 2 Caption: A different version of the pie chart with slightly different sets of values.

6. Dark energy makes up 74 % and dark matter makes up 22%. Baryonic matter (i.e., ordinary matter consisting of protons, neutrons, and electrons) makes up only 4% of the mass-energy of the observable universe. And most of the baryonic matter is nearly invisible baryonic dark matter which is mostly intergalactic medium (IGM) (made of hydrogen and helium gas and NOT including any dark matter).

   Only 0.4 % is of the mass-energy of the observable universe is stellar matter---which is baryonic matter which is NOT baryonic dark matter.

   Note stellar matter consists of:

   1. stars.
   2. interstellar medium (ISM) which in turn consists of a) interstellar gas of hydrogen, helium, and a few percent or less metals and b) interstellar dust (which is all metals).
   3. compact remnants and brown dwarfs.

   The fact that stellar matter makes up so little of the mass-energy of observable universe is quite a change from circa the 1970s when some folks maintained that it was still possible to hypothesize that it was all of mass-energy by discounting the still controversial evidence for dark matter.

   The baryonic dark matter is, as aforesaid, mostly intergalactic medium (IGM) (which is intergalactic gas). The IGM has very low density, but there is a lot of space between galaxies, and so a lot mass in the IGM. Much of the IGM warm-hot intergalactic medium (WHIM)). WHIM is nearly invisible since it only radiates a little in the X-ray. Since the 2010s, we do have had significant observations of WHIM.

7. Why do we need dark matter and what is dark matter?

   To account for the structure (including motions) of galaxies, galaxy groups, galaxy clusters, galaxy superclusters, we need far more mass-energy than is seen in stellar matter and this extra mass-energy is far more than allowed by the highly successful theory of Big Bang nucleosynthesis.

   Hence there must be dark matter.

   What are theories for what for dark matter and its theoretical replacement?

   1. The favored theory is that dark matter is an exotic elementary particle and there many theories about exactly what kind of elementary particle it is. However, it certainly interacts weakly with itself and baryonic matter, except via gravity.

      The dark matter particles just fly around as a nearly pressureless gas.

      The dark matter clumped into dark matter halos under self-gravity and the dark matter halo gravity pulled baryonic matter into the dark matter halos, and so initiated the formation of the large-scale structure of the universe in the reionization era (AKA cosmic dawn: cosmic time ∼ 0.150--1 Gyr, z∈∼[6,20]).

      In gravitationally-bound systems, the dark matter particles must individually follow chaotic orbits and NOT move as clumps of matter the way an ordinary gas with pressure would.

      In fact, there is some hope that the dark matter particle will be discovered in circa the 2020s---but hopes have been dashed before.

   2. Another idea is that dark matter is primordial black holes (PBHs).

      The PBHs act just like the just described dark matter particles in cosmic evolution.

      For more on PBHs as dark matter, see Black hole file: black_hole_primordial.html.

   3. A third idea is that there is NO dark matter and our theories of gravity (Newton's law of universal gravitation and general relativity (GR)) both need revision and the revised versions will account for what dark matter is used to account for. The leading revised theory is called MOND (MOdified Newtonian Dynamics).

   4. Occam's razor disfavors this, but it may be that we live in the worst of all possible worlds where we have all three solutions: dark matter, PBHs, and MOND.

8. The dark energy, even more than dark matter, is a name that covers our ignorance.

   There is no consensus theory of what it is.

   We see an effect, acceleration of the universe (i.e., the observed acceleration of the expansion of the universe), and define dark energy as the cause.

   The simplest dark energy is the cosmological constant Λ (pronounced Lambda) which is NOT really an energy at all, but a modification of gravity in general relativity. It is the simplest of all modifications to general relativity to get the effect of acceleration of the universe, and so is favored by Occam's razor over other modifications. In fact, astronomers often just say Lambda as synonym for dark energy.

   The cosmological constant Λ is the origin of the Λ in the name Λ-CDM model.

   However, dark energy may be a real form of energy. The simplest theory is that it acts just like the cosmological constant Λ in causing the acceleration of the universe, but quantum field theorists think a real dark energy should have other properties.

   Such properties could include nonconstancy in cosmic time and space and interactions with other forms of mass-energy other than gravity.

   However, there is NO consensus theory of what the nonsimplest dark energy should be like.

   Hopefully, new observations circa the 2020s will elucidate the dark energy.

Images:

1. Credit/Permission: NASA, before or circa 2007 / Public domain.
   Image link: Wikipedia: File:DarkMatterPie.jpg.
   
2. Credit/Permission: NASA, before or circa 2006) (uploaded to Wikimedia Commons by User:Rogilbert~commonswiki, 2006) / Public domain.
   Image link: Wikipedia: File:Cosmological composition.jpg.