Kelly McSweeney

Jan 27th 2020

Dark Energy vs. Dark Matter: What’s the Difference?


According to NASA, less than 5% of the universe is made of normal matter. That means everything we can observe from Earth is just a tiny fraction of reality.

The rest is dark and unknown; 68% of the universe is dark energy and 27% is dark matter. Two of the biggest mysteries of the universe aren’t nearly as related as their names imply. Dark energy is a property that pushes things apart in space, while a dark substance pulls celestial objects toward it. We don’t know much about either one, although experiments could find the answers, and you never know when scientists will have a breakthrough that would fundamentally change our understanding of the universe.

Dark Matter

There is a physical substance in space that we can’t see, although astronomers can observe its influence. The force of gravity from all visible matter isn’t strong enough to keep a galaxy together, so there must be something else that has mass that we can’t see. A dark and mysterious form of matter fills out the missing parts of the equation. This invisible substance produces an attractive force (gravity) that pulls matter inward and helps hold galaxies together, according to Mental Floss.


Previously, astronomers expected the stars at the center of our galaxy to move faster than those on the galaxy’s edge, according to the Hobby Eberly Telescope Dark Energy eXperiment (HETDEX). This seems logical because the center of the galaxy, which is the center of the mass, should have the strongest gravity.

But when astronomers measured the stars that form a bright disk across the Milky Way galaxy, they were surprised to discover that all stars orbit the center of the galaxy at about the same speed. There’s some other invisible matter pulling at the stars from the outside.

Astronomer Fritz Zwicky discovered the substance in 1933, according to Britannica, when he observed that visible matter didn’t provide nearly enough mass to keep the stars from flying off into space. Astronomers have since observed this effect in other galaxies, as well as clusters of galaxies. Calculations now show that a vast “halo” of a dark substance surrounds the Milky Way. This halo may be 10 times as massive as the bright disk, so it exerts a strong gravitational pull.

What Is It Made of?

We still don’t know exactly what it is, but scientists have a few working theories. In the meantime, according to NASA, we’re working on process of elimination. Here are some things that it is not:

  1. Visible matter (we can’t see it).

  2. Dark clouds of normal matter made of particles called baryons. Scientists would be able to detect baryonic clouds because they would observe radiation passing through them.

  3. Antimatter. Scientists do not see the unique gamma rays that are produced when antimatter annihilates with matter.

  4. Galaxy-sized black holes. Astronomers say the gravitational lenses that they see eliminates this possibility.

So, what does that leave us? Some possible candidates are dense astronomical bodies called massive astrophysical compact halo objects (MACHOs) or weakly interacting massive particles (appropriately nicknamed WIMPs), according to Mental Floss.

Dark matter likely consists of an exotic subatomic particle that hasn’t been detected yet. Scientists are conducting experiments at the International Space Station and CERN to try to capture these elusive particles, although nobody has succeeded yet.

Dark Energy

While matter attracts (gravity), energy, on the other hand, produces a repulsive force (antigravity) that pushes matter outward. It’s not an object but rather a property of space, and it helps explain why our universe is expanding. According to HETDEX, dark energy shows itself only on the largest cosmic scale.


In 1929, legendary astronomer Edwin Hubble observed the “redshift” phenomenon that proved that our universe has been expanding since the Big Bang. For decades, astronomers believed that gravity would cause the expansion to slow down. Then in 1998, astronomers were shocked to learn the opposite.

According to Mental Floss, astronomers were using the Hubble Space Telescope to study distant supernovae, when they observed that around 7.5 billion years after the Big Bang, the universe began expanding at an even faster rate. According to NASA, no one expected to discover that the expansion was accelerating, and scientists still can’t explain it, but they can tell that it exists because it affects the expansion of the universe.

What Is It Made of?

According to NASA, there are several theories for what dark energy is. It may be an inherent property of space. Contrary to popular belief, empty space is not nothing. It has properties; it can expand and possess its own energy, as Albert Einstein suggested with his gravity theory and cosmological constant.

The quantum theory of matter provides a second explanation. This theory suggests that “’empty space’ is actually full of temporary (‘virtual’) particles that continually form and disappear.” However, this theory is highly unlikely, because when physicists attempted to use this theory for calculations, the answer came out wrong, according to NASA.

A third option is that dark energy is a new kind of dynamical energy fluid or field, which some scientists have called “quintessence.” If that’s the case, we still don’t know anything about it, says NASA.

Lastly, perhaps Einstein’s theory of gravity is not correct, and that’s why dark energy doesn’t fit into our equation of the universe. All of our current knowledge and understanding of the universe depend on Einstein’s theory being true, so if dark energy exposes an error, then we’re back at square one. It would upend our entire understanding of the universe.

Mysteries Remain

All this is to say, we still don’t know much about these “dark” aspects of space. One is a thing, another is a property, and they are both invisible and illogical. That’s why dark matter and dark energy are two of the most exciting areas of science, and scientists around the world are racing to find the answers to the dark parts of space. At this point, what we know is a process of elimination and circumstantial evidence. We know what they’re not, we see the effects, and there is evidence that they exist.

Space is often seen as the ultimate symbol of nothingness, but in reality, space isn’t empty at all. It’s just full of dark things and dark forces that we don’t understand yet.