A fundamental question scientists have been asking about the expanding universe is whether it will continue to expand forever. Since the big bang occurred billions of years ago, galaxies formed, evolved, and have been flying deeper and deeper into space. If there is enough total mass in the universe, its gravitational attraction will slow the expansion, reverse it, and eventually result in a "big crunch." If the mass is insufficient, the expansion could continue forever. Much of the total mass of the universe may not be detectable by ordinary instruments such as a telescope--hence the search for dark matter which would tell cosmologists where the universe is headed.

In 1933 Swiss astronomer Fritz Zwicky observed that some galaxies in a cluster had a higher speed than expected. It was surmised that invisible dark matter was gravitationally affecting their movement. His measurements showed there had to be more...