To the Editor:

In focusing on scientific explanations for dark energy (“Quest for Dark Energy May Fade to Black,” Jan. 4), you overlook a possible social explanation.

It seems implausible that human beings represent the only advanced civilization in the universe (if I may flatter us by calling us advanced). If indeed we are not alone, then, as Fermi asked, “where are the aliens?”

My modest proposal is that the aliens are the cold dark matter that makes up 23 percent of the energy in the universe. Adept at cloaking themselves, they are invisible under any force that can be shielded, including strong, weak and electromagnetic interactions. Absent a breakdown in their cloaking mechanism, we are unlikely to detect them directly.

The gravitational force cannot be shielded, however. Whatever they are made of, the aliens cannot prevent their mass from altering the rotation curves of the galaxies they inhabit.