Hundreds of rapidly moving galaxies often cluster tightly together. Their relative velocities, as inferred by the redshifts of their light, are so high that these clusters should be flying apart, because each cluster’s visible mass is much too small to hold its galaxies together gravitationally.a Because galaxies within clusters are so close together, they have not been flying apart for very long.
A similar statement can be made concerning many stars in spiral galaxies and gas clouds that surround some galaxies.b These stars and gas clouds have such high velocities that they should have broken their “gravitational bonds” long ago—if they were billions of years old. If redshifted starlight always indicates a star’s velocity, then a multi-billion-year-old universe is completely inconsistent with what is observed.
These observations have led some to conclude, not that the universe is young, but that unseen, undetected mass—called dark matter—is holding these stars and galaxies together. For this to work, about 85% of the mass in the universe must be invisible—and hidden in the right places. However, many experiments have shown that the needed “dark matter” does not exist.c Some researchers are still searching, because the alternative is a young universe. [See "Dark Thoughts" on page 33.]