Dark Matter & Dark Energy

Dark Matter:

Originally known as the “missing mass,” dark matter's existence was first discovered by Swiss astronomer Fritz Zwicky, in 1933. Fritz discovered that the mass of all the stars in the cluster of galaxies provided only about 1% of the mass needed to keep the galaxies from escaping the cluster's gravitational pull. When this was first published it was largely ignored for over 40 years. Until late 1970s, when astronomer Vera Rubin, with the help of her associate Kent Ford, found that the stars moved at similar speeds throughout the galaxy, distance from the core had little effect on their velocity. When she subsequently measured stars in 200 other galaxies, she obtained similar results. 

She then recalled Zwicky's observations and realized the missing mass he predicted could also explain the unexpected star velocities she observed. Initially, many astronomers disbelieved her, too. However, her observations were repeatable and the explanation she offered was completely unambiguous so, within a few years, Rubin's findings became an established concept in modern astrophysics. 

Up until this day we are not sure what dark matter is, but we are damn certain what dark matter is not. First, it is dark, it does not emit nor does it reflect light, it is not in the form of stars, planets big black holes … nothing we see with our eyes or detect with our instruments. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons, we can detect baryonic clouds by their absorption of radiation passing through them. Third dark matter is not anti-matter because anti-matter produces unique gamma rays when it reacts with normal matter. Finely dark matter is not large galaxy-sized black holes —very compact objects that violently effect their surroundings—  while dark matter is scattered everywhere.

However, at this point, the two main categories of objects that scientists consider as possibilities for dark matter include MACHOs, and WIMPs.

MACHOs (MAssive Compact Halo Objects): MACHOs are objects ranging in size from small stars to super massive black holes. MACHOS are made of ordinary matter (like protons, neutrons and electrons). They may be black holes, neutron stars, or brown dwarfs.

WIMPs (Weakly Interacting Massive Particles): WIMPs are the subatomic particles which are not made up of ordinary matter. They are "weakly interacting" because they can pass through ordinary matter without any effects. They are "massive" in the sense of having mass (whether they are light or heavy depends on the particle). The prime candidates include neutrinos, axions, and neutralinos.

Dark Energy:

Albert Einstein was the first person to realize that empty space is anything but empty. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe.

we have multiple ideas about what dark energy might be, one idea is that "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong ( very very wrong ). It is hard to get a worst answer so the mystery continues.

Another idea is that dark energy is some kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. But if this theory is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.

Another idea comes from Einstein's gravity theory, the version with the cosmological constant, which predicts that dark energy is just a property of space, and as more space comes into existence, the stronger the dark energy becomes, resulting in the ever accelerating expansion of the universe.  Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.

A last possibility is that Einstein's theory of gravity was wrong all along, and we need a new theory of gravity. So far there are candidate theories, but none are compelling. So the mystery continues.