What is the universe made of? If you answered stars, planets, gas and dust, you’d be dead wrong. Thirty years ago, scientists first realized that some unknown dark substance was affecting the way galaxies moved. Today, they think there must be five times as much dark matter as regular matter out there. But they have no idea what it is — only that it’s not made of atoms, or any other matter we are familiar with. And Dark Matter is not the only strange substance in the Universe — a newly discovered force, called Dark Energy, seems to be pushing the very fabric of the cosmos apart.
What is the Universe Made of?
By Chelsea Hedquist, HowStuffWorks.com
More Than Meets the Eye
The composition of the universe may seem straightforward, something you mastered back in your junior high science class — galaxies made up of planets and stars, stars made up of burning gases and dust. But this idea of the universe only includes the parts that we can see, either with the naked eye or even with powerful telescopes. According to scientists, the visible portions of the universe account for less than 95 percent of what is actually out there in the great expanse of space. Much of the universe is made up of something we can’t see. We call this something “dark matter,” and we only discovered its existence because something else was missing.
As they study the universe, astronomers often compute the mass of galaxies to help them estimate the mass of the universe, or how much matter exists in it. They do this by measuring the light reaching Earth from distant galaxies and use this measurement to extrapolate how many stars — and consequently how much matter — the universe contains. Scientists can also use a spectroscope to measure the Doppler shift, allowing them to determine how fast galaxies are rotating. With this information, they can calculate the mass of these galaxies.
In 1933, a Swiss astronomer named Fritz Zwicky made a surprising discovery while using these methods to compute the mass of a cluster of galaxies. When he used the galaxies’ rotational velocity to measure mass, he came up with a figure that was 400 times larger than the figure he came up with when he measured mass based on the galaxies’ light (or matter that he could observe). The inconsistency between calculations gave rise to what scientists today call “the missing mass problem.”
So the universe must be filled with much more matter than what can actually be observed. Scientists have termed this mysterious, unseen part of the universe “dark matter” and, despite years of research, they still understand relatively little about it.
Although scientists are still struggling to determine what dark matter is, precisely, they at least have some ideas about what it may be — more than they did back in the 1990s when some astrophysicists admitted they couldn’t find the bulk of the universe. For starters, they know that some of it might be ordinary matter, composed of protons, neutrons and electrons. In fact, it’s the kind of matter that we humans are made of. Astronomers call this baryonic matter. In the case of dark matter, it may come in one of a few (dimly lit) forms nicknamed massive compact halo objects (MACHOs) such as brown dwarfs, white dwarfs or black holes. But so far astrophysicists think these objects are too rare to solve the missing mass problem.
Instead, it seems likely that dark matter largely comprises “extraordinary matter,” new forms of matter composed of new types of particles. Cosmologists hypothesize that dark matter may actually be made up of particles that were produced shortly after the big bang. Scientists have dubbed these subatomic particles WIMPs (weakly interacting massive particles). WIMPs may be part of a supersymmetry of particles — meaning each known particle has a “superpartner,” such as axions and neutralinos, which many scientists point to as probable candidates for making up dark matter.
Recent observations have led to the important conclusion that, while the Earth may be round, the universe is actually flat. But scientists estimate that dark matter makes up only about 23 percent of the mass necessary to produce a flat universe, leaving more than 70 percent of the necessary mass still unaccounted for. Today, NASA scientists hypothesize that 72 percent of the universe is composed of dark energy, a different substance that we know even less about. This substance exerts a negative pressure on the universe, causing it to expand at an accelerated rate.
Much remains unknown about dark matter and dark energy — two substances unlike anything previously recognized in our universe that nevertheless make up the vast majority of it. But new advances in technology used to detect dark matter, as well as particle accelerator experiments, will likely bring scientists closer not only to answering the question “What is the universe made of?” but also to actually understanding that answer.
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