If we were to add up all the mass of every person, house, planet, and galaxy in our universe, we would only end up with about five percent of the universe’s total mass. So where is the other 95%?

Well, physicists and astronomers theorize that the remaining mass of the universe is taken up by two things: Dark matter and Dark Energy. Dark matter makes up about 27% of the universe’s remaining mass, while dark energy takes up the remaining 68%. So what is Dark Energy, and why does it occupy most of our universe?

To understand dark energy I’m going to have to take you back… way back to the origin of our universe, The Big Bang. Ever since then, our universe has been expanding, but about 4 billion years ago the rate at which the universe expands began to accelerate. Up until the 2oth century, the universe’s rate of expansion was actually thought to be slowing down; however, we are able to prove the opposite through the Doppler effect and redshift. The Doppler effect states that the frequency of a wave changes as it moves either closer to or further from an observer. As the source of a wavelength approaches an observer, its wavelength is shorter and has a higher frequency. Vice Versa, when the source is further away, its wavelength is longer and has a lower frequency. Think of it like this! When you hear an ambulance approaching you, the pitch of the sirens becomes much higher; as a result, having a shorter wavelength and a high frequency. When the ambulance is moving away from you, the pitch lowers showing you its low frequency due to a longer wavelength. With light waves, we are able to see this same effect in outer space. Short visible light waves, present a bluish color, so galaxies and stars closer to us emit a red hue. On the contrary, longer and further away light waves emit a red hue. When observing distant galaxies, it was determined that the light waves emitted from these distant galaxies were so much further to the red extreme of the visible light spectrum that they must be moving away from the Milky Way Galaxy. This is known as the redshift.

So, what does this have to do with dark energy? Well, dark energy is the cause of the acceleration of the expansion of the universe. In other words, dark energy is what makes the universe expand faster and faster. In this case, dark rather means unknown… not actually dark because well, we don’t know that much about it. We do know, however, that the total amount of dark energy increases as the volume of dark energy increases with time.  We also know that dark energy should be thought of as a property of space that keeps pushing galaxies further away from each other.

Well, what don’t we know? Basically everything else, but that’s what makes the future of science so exciting.