Continued Expansion

What will happen to the universe in the distant future? Will the galaxies rush apart forever, or is there enough gravitational attraction to pull the galaxies back together? The answer depends on the mass of the entire universe. To have a gravitational force strong enough to reverse the expansion, there must be sufficient mass in the universe. If there is less than this amount of mass, the universe will continue to expand.

How can the amount of matter in the universe be determined? Some matter, like stars, glows. The mass of matter can be seen. However, much of the matter in the universe can't be seen. Dark matter is matter that does not give off radiation. Dark matter cannot be seen directly, but its presence can be detected by observing its gravitational effects on visible matter. American astronomer Vera Rubin found that galaxies like ours may contain as much as ten times more dark matter than visible matter. Without this dark matter, there would not be enough gravitational force to keep groups of galaxies from flying apart.

There are many unanswered questions about dark matter. Astronomers don't know what it is made of or how it is distributed in the universe. Some astronomers hypothesize that dark matter consists of WIMPs, which stands for weakly interacting massive particles. Currently, experiments are underway to detect WIMPs.

In the past few years, astronomers have discovered that the rate of expansion of the universe may be increasing. Galaxies appear to be moving apart faster now than expected. The reason for this is uncertain. A mysterious force called dark energy is theorized to be causing the rate of expansion to increase. If the expansion is accelerating, it's likely that the universe will expand forever.

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