What is the universe made of?
The visible matter in the universe that makes up the familiar structures of the galaxies -- is composed in approximately the following manner:
- Hydrogen: 72-75%
- Helium: 23-26%
- All Other Elements: < 2
While the process of nuclear fusion has very slightly increased the quantities of helium -- and produced almost all of the other elements present -- these figures are practically unchanged since the earliest years of our universe.
The vast majority of the material in the universe -- perhaps as much as 90 percent -- is not visible. This so-called "dark matter" reveals itself only through its gravitational effect on other objects, such as galaxies in large clusters. Astronomers, faced with the complete mystery of such an enormous fraction of the universe's constituent material, have proposed a number of unusual possibilities, from weakly interacting massive particles (WIMPs) such as neutrinos, to massive compact halo objects (MACHOs) such as brown dwarfs. Results from studies of these options remain inconclusive, however, and the true nature of "dark matter" -- and what most of the universe is made of -- remains a question.
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