Question: We now hear the terms “dark matter” and “dark energy.” What are these? (Asked by a curious space cadet.)

Reply: These are fairly new terms in astronomy or cosmology, and are pretty involved and theoretical. I’ll do my best to explain them.

Analysis of the orbital speed of stars in the Milky Way indicates that it contains much more matter that is accounted for by the stars and gas in it. (Orbit speed is proportional to the mass of the orbiting body.) Almost everywhere astronomers look — the Milky Way, other galaxies or a cluster of galaxies — most of the matter needed to cause the galaxy’s gravitational behavior (such as the orbital speed of stars) seems to be unseen by telescopes using light, radio waves, X-rays or any other electromagnetic radiation.

Oddly enough, it appears only about 5% of the universe is accounted for as normal matter. The unseen matter was originally referred to as missing mass, but a better term, “dark matter,” is now used. (Dark because we cannot see it.) Apparently, the disk of the Milky Way is embedded in a “halo” of dark matter that extends perhaps 150,000 light-years from the Milky Way’s center.

Regardless of how scientists figure it, even though gravity is pulling inward, the universe keeps expanding outward faster and faster. To account for this, it is proposed that something invisible counteracts gravity by pushing the universe apart. This is called “dark energy,” which gives rise to a negative pressure that drives space apart. As space expands, more space is created, and with it, more dark energy. Based on the observed rate of expansion, scientists estimate that the sum of all the dark energy must make up more than 70% of the total contents of the universe.

However, no one has been able to put a finger on it. All this is bound up in modern, relativistic cosmology — a tough subject.

C.P.S. (Curious Postscript): “It always looks darkest just before it gets totally black.” — Charlie Brown

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or email Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to