Dark matter enlightened
Tom Siegfried’s article on dark matter “Light in the Dark,” (SN: 1/12/13, p. 18) reminded me of the 19th century search for the luminiferous ether. One can only wonder if history will repeat itself in the 21st century search for dark matter.
Jeffery Miller, Los Angeles, Calif.

The difference is that the ether was only surmised; attempts to observe its effects failed. There is abundant observational evidence for the existence of dark matter. Its actual identity could still, of course, turn out to be very surprising. — Tom Siegfried

Where did the figure that 17 percent of the universe is composed of ordinary matter come from? The data I have seen is that ordinary matter constitutes less than 5 percent of the universe, 23 percent is dark matter and 72 percent is dark energy.
Dennis Blanchard, San Jose, Calif.

About 72 percent of the total mass plus energy content of the universe is dark energy; nearly all of the remaining 28 percent is matter of some form, either ordinary atomic matter or the mysterious dark matter, which is distinct from dark energy. By the latest calculations, about 4.6 percent of the total mass-energy is ordinary matter, which comes to a little less than 17 percent of the total amount of matter. — Tom Siegfried

Why is it assumed that dark matter exists in our part of the galaxy? If WIMPs cause the edges of galaxies to rotate much faster than predicted by gravity, wouldn’t that imply that they are distributed differently than ordinary matter? For example, they might be confined to a halo surrounding the galaxy.
Bob Eramia, Seattle, Wash.

Scientists have assumed Earth should have more interactions with WIMPs when moving in the direction of the constellation Cygnus. I assume that is because our solar system is moving in that direction within the Milky Way and not simply because of our revolution around the center of the galaxy.
Sam Scrutchins, Benicia, Calif.

“Halo” is a misleading term. If the theory is correct, the galaxy sits embedded in a vast cloud of dark matter which extends well beyond the visible part of the galaxy but also is present within the galaxy, including the solar system. The dark matter particles should be striking the Earth from all directions. The Earth and sun’s movement toward Cygnus is a result of the galaxy’s rotation; that motion means more impacts with dark matter particles would be expected from that direction. — Tom Siegfried