The Composition of the Universe

Stars and galaxies, such as the Andromeda Galaxy and our own Milky Way, only make up a tiny part of the mass of the universe. What else is there?. Adam Evans/Wikimedia Commons.

What makes up the universe? That's a question that astronomers are still answering. You'd think it would be pretty easy to figure out, since astronomers know about stars and galaxies and can measure their masses pretty well. So, how difficult can it be to look out into the cosmos and estimate, with reasonable accuracy, the amount of luminous mass (the material we can see) in the universe,using radio, infrared and X-ray astronomy?

Its not that difficult and astronomers are making great advances in those measurements. But that's not the problem. The answers they're getting don't make sense. Is our method of adding up the mass wrong (not likely) or is there something else out there; something else that we can't see?  Let's take a look at how we measure cosmic mass.

Measuring Cosmic Mass

One of the greatest pieces of evidence for the mass of the universe is something called the cosmic microwave background (CMB). It dates back to shortly after the Big Bang and is actually the background temperature of the universe. It's a heat that is detectable throughout the cosmos equally from all directions, and is measured at 2.7 degrees K. There are small, but important fluctuations spread throughout this "heat" measurement, however its even distribution indicates that the universe must be nearly flat - essentially without curvature.

So, what does that mean for measuring mass? Essentially, given the measured size of the universe it means there has to be a particular amount of mass and energy present within it to make it "flat".

The problem? Well, when we add up all of the "normal" matter (such as stars and galaxies, plus the gas in the universe, that only gives us about 5% of the critical density to explain a flat universe.

So where is the other 95%? Scientists say that it exists as dark matter and dark energy

The Composition of the Universe

So, there is mass that we can see (what we call "baryonic" matter) and also mass that we can't (the dark matter). There is also energy (light) that we can measure and energy that we can't (the dark energy). Here's a breakdown of the current proportions of mass in the universe:

  • Heavy elements (~0.03%): For nearly half a billion years after the birth of the universe the only elements that existed were hydrogen and helium. It wasn't until stars were born, lived their lives, and then died that heavier elements were created.  That happens as stars fuse hydrogen (or other elements) in their cores. Stardeath spreads all those elements to space through planetary nebulae or supernova explosions. After this event the elements would be spread out in the region, to be used up in the formation of other stars, planets and other objects. This is a slow process though and even nearly 14 billion years after its creation, the only a small fraction of the mass of the universe is in elements heavier than helium.
  • Neutrinos (~0.3%): Created during the nuclear fusion process in the cores of stars, neutrinos are nearly massless particles that travel at nearly the speed of light. Couple with their lack of charge, this tiny mass means that they do not interact readily with mass except for a direct impact on a nucleus. Measurements of neutrinos - a non-trivial task - has led to an estimation of nuclear fusion rates of our Sun and other stars, as well as an estimate of the total neutrino population in the universe.
  • Stars (~0.4%): When we peer out into the night sky most of what we see is stars. In fact, when we look at the visible light coming from other galaxies even, most of what we see are stars. However, in spite of this, stars actually make up only a very small portion of our universe.
  • Gas (~4%): More abundant than stars is the stuff that usually occupies the space between stars, and for that matter, the space between whole galaxies. Interstellar gas, which is mostly just free elemental hydrogen and helium makes up most of the mass in the universe that we can directly measure. We don't normally "see" gas in the usual sense (i.e. looking through a telescope). Rather, we detect the presence of gas using instruments sensitive to radio, infrared and x-ray wavelengths.
  • Dark Matter (~22%): Analyzing the motion (rotation) of galaxies, as well as the interaction of galaxies in galaxy clusters, researchers have found that all of the gas and dust present is not enough to explain what we see. Rather, 80% of the mass in these galaxies must be "dark"; not detectable in any wavelength of light, radio through gamma-ray. The identity of this mysterious mass? Unknown. The best candidate is cold dark matter, which is theorized to be a particle similar to a neutrino, but with a much greater mass. It is thought that these particles, often known as weakly interacting massive particles (WIMPs) arose out of thermal interactions in early galaxy formations. However, as yet we have not been able to detection dark matter, directly or indirectly, or create it in a laboratory.
  • Dark Energy (~73%): And, finally, the substance that is to make up the greatest percentage of stuff in the Universe, is by far the least understood. In fact, dark energy isn't (likely) even massive at all. It is possibly a very strange property of space-time itself, or maybe even some unexplained (so far) energy field that permeates the entire universe. Or its neither of those things. We don't know. Only time and lots and lots more data will tell.