I am surprised at the matter-of-fact sounding statements in this article: “Dark matter reacts only to gravity. Unlike visible matter, it can’t be pushed by winds.” We hypothesize the existence of dark matter to explain observations that could be attributed to gravitational forces, but we don’t know what dark matter might be. How can one say that it reacts only to any one force and that it can’t be pushed by winds?

Clinton C. Brooks
Annapolis, Md.

Scientists can’t be sure that any particular form of dark matter exists until they detect it, but if properties of dark matter were very different than envisioned, the arrangement of galaxies wouldn’t be as it is today. –R. Cowen