Trying to determine the difference between dark matter and dark energy is very tricky to scientists because they have no way of knowing if they are simply manifestations of the very same phenomena. Scientists know that both dark matter and dark energy must exist in order to account for certain observations that have been made in the past, but they do not know very much about their make up so there is no scientific reason to assume they are connected. Therefore, for the time being, science is treating them as separate entities.
It is presently believed that outer space is made up of approximately 0.03% of heavy elements (which means any element besides helium and hydrogen), 0.5% stars, 0.3% neutrinos, 4% free hydrogen and helium, 70% dark energy, and 25% dark matter. Thus, even though scientists know little about dark matter and dark energy, they are quite certain that both of them make up over 90% of the universe – meaning it is pretty important stuff.
The best way to distinguish between dark matter and dark energy is by learning what scientists know about each of them individually.
What is Dark Matter?
Dark matter is a very bizarre ingredient which doesn’t emit any light or energy. Its presence was first postulated during the 1950s while studying neighboring galaxies. Data indicated that the universe must consist of more matter than previously believed. This belief has picked up more and more support from the scientific community over the years. Its strongest support has come in recent years.
Space researchers claim that the motions of the stars will always indicate the amount of matter the universe contains. And even though there is no direct evidence for dark matter, the stars tell us that it is there. Recently, a galaxy known as Dragonfly44 has been identified as being composed pretty much entirely of this dark matter.
Scientists can determine the mass of large celestial objects by studying how they move. Astronomers that examined spiral galaxies during the 1950s expected to objects in the center of those galaxies to move faster than those that were on the outer edges. Instead, they discovered stars at both locations were traveling at the same speed, which indicated the galaxies had much more mass than they could see. In addition, studies of gases within elliptical galaxies also noted a lack in mass for what they were observing visually. These galaxy clusters would quickly fly apart if they only contained the mass that was visible by conventional astronomical measurements.
The material in the universe that scientists are familiar with is known as baryonic matter. And baryonic matter consists of neutrons, protons, and electrons. However, dark matter could be made from this baryonic matter or it could be made of non-baryonic matter. We simply do not know at the present time. But in order to hold our universe together, dark matter must make up a healthy percentage of its overall matter.
The majority of scientists believe that dark matter is made up of the non-baryonic matter. This actually makes the missing matter even more challenging to find. Potential places to find the missing matter are places like white dwarfs, dim brown dwarfs, along with neutrino stars. It is also believed that supermassive black holes could make up the difference as well. But others believe that hard to find objects like these would need to play a much larger role than what has been observed to account for all this missing mass. This implies that dark matter could be more exotic.
One of the lead candidates for exotic matter are weakly interacting massive particles (WIMPS), which have anywhere from 10 to 100 as much mass as a proton, but their “weak” interactions with normal matter make them very hard to detect. Neutralinos, which are totally hypothetical particles that are slower and heavier than neutrinos, are the most likely candidate, but they haven’t yet been spotted.
One more potential source of dark matter is sterile neutrinos. Neutrinos are particles that are not a part of regular matter. A constant flow of neutrinos come from the sun, but since they hardly ever interact with normal matter, they actually pass right through the Earth and all of its inhabitants. There are presently three (3) kinds of neutrinos. But a recently added fourth type of neutrino is called the sterile neutrino has been proposed as a potential dark matter candidate. This new sterile neutrino would interact with normal matter only through gravity.
What is Dark Energy?
The straightforward answer to this question is that we are not sure about the composition of dark energy. We have some pretty good ideas but dark energy still seems to contradict a lot of our previous beliefs about the manner in which the universe operates.
We do know a light wave, which is also referred to as radiation, carries energy. We instantly feel this energy on hot summer days. Albert Einstein’s quite famous equation, E = mc2, has taught us that energy and matter are interchangeable, simply different forms of the very same thing. We happen to have a stunningly huge example of this in our daily sky – our own Sun. The Sun derives its power by converting mass to energy.
The thing about energy is that it must have a source, either in the form of radiation or matter. What we have observed about the universe is that it has residual energy, even if it were devoid of all radiation and matter. This energy of space on a huge cosmic scale carries with it a force that actually increases the expanding of the universe.
Many believe that dark energy can be described from the strange behavior on a scale that is smaller than an atom. The physics of this very small world is referred to as quantum mechanics, which permits matter and energy to pop into nothingness out of the blue, even for just an instant. This flux of constant, yet brief disappearance and appearance of matter could very well be providing energy to an otherwise empty space.
Another interesting theory is that this dark energy has created a brand new, fundamental force in our universe, a force that only reveals itself whenever the universe attains a certain size. There are already some scientific theories out there that allow for the potential of such forces. And it could be that this new force is temporary, causing acceleration of the universe for a few billions years before it grows weaker and eventually disappears again.
Or it could be that the mystery of dark energy lies within yet another unsolved problem that has been around for a while, and that is how to reconcile the physics of our larger world with the physics of the smaller atomic world. Albert Einstein’s theory of gravity, which is called general relativity, explains everything about planet movements all the way to the activities of black holes, but it does not apply on the tiniest scale regarding particles that make up the atomic world.
In order to forecast how these tiny particles behave, we use the theory of quantum mechanics. Quantum mechanics attempts to explain the way that tiny particles function, but it just does not apply to anything larger than atoms. The solution we seek about dark energy might be found in the combining of these two popular theories.
Dark Matter and Dark Energy Summary
Now let’s summarize the difference between Dark Matter and Dark Energy.
Dark Matter has to exist in order to account for the mass needed to hold galaxies together. Newton’s Laws taught us that mass determines the amount of gravity within a solar system. The mass we observe in distant galaxies is simply not enough hold them together – dark matter must exist to supply the needed gravity.
Dark energy has to exist to account for rate of expansion we are observing in our universe. Not only is our universe presently expanding, the rate of expansion is accelerating and this ‘anti-gravity’ force that is at work is referred to as ‘dark energy’.
There are several researchers out there who are presently searching to find an explanation that will encompass both dark energy and dark matter. It is always very desirable to be able to explain more than one phenomenon with just one theory or law of science. But there are many other researchers who strongly believe that these two entities are distinct and we are trying to solve two different problems. For instance, several string theories have used supersymmetric particles to account for dark matter, yet they make no link to dark energy whatsoever. Time will eventually reveal the solution.