Dark and darker: there's a lot more gravity in the cosmos than meets the eye

What if all the dark matter during the first few minutes after the big bang had been dark ordinary matter? Six times as many interactive particles would have been squeezed into the infant universe. That huge extra quantity of ordinary matter, gung-ho as it was for fusion, would have dramatically pumped up the fusion rate of hydrogen but would have consumed much of the deuterium in the process.

As it happens, all the deuterium in the cosmos was manufactured immediately after the big bang. And because deuterium is readily consumed in the cores of stars, the most un-evolved regions of the cosmos should hold no more of the stuff than existed at the end of the fusion era, after the first few minutes of the universe. And sure enough, the spectra of galaxies whose gas clouds have been only minimally processed show one deuterium atom in every hundred thousand. Just what one would expect from a big-bang birthday suit wrapped in a dark-matter blanket.

Astrophysicists are generally reluctant to base calculations on things they don't understand. Unrelenting skeptics will surely compare the dark matter of today with the hypothetical, now-defunct "luminiferous aether" that generations of physicists had proposed as the carrier of light waves. But dark-matter ignorance is fundamentally different from aether ignorance. Whereas the aether was a cover for scientific cluelessness, dark matter is not merely presumed to exist. Its gravity has been shown to exist. No one is pulling it out of thin space. In fact, dark matter is no less real than the hundred-plus planets now known to orbit stars other than the Sun--planets discovered solely by the effects of their gravity on their central star.

Other unrelenting skeptics might declare, "Seeing is believing." That premise may make you a good carpenter or cook or resident of Missouri, but it won't make you a good physicist. Physics is not about seeing; it's about measuring--preferably with something that's not your own eyes, which are inextricably conjoined with the baggage of your brain.

Having resisted all attempts to understand it, dark matter has become a kind of Rorschach test for investigators. Gravity skeptics say we don't really understand gravity. Particle physicists say dark matter could be some ghostly class of undiscovered particle. Maybe the particles interact through gravity plus some unknown additional force, or maybe (and more likely) they do respond to normal forces, but so weakly that the particles are virtually undetectable. Or how about the cosmic exotica that sometimes appear in "theories of everything"? Perhaps a parallel, phantom universe exists right next to ours, revealed only through its gravity. We'll never actually run into its matter, but we might feel its tug.

Whatever dark matter is, though, its effects in the contemporary universe are straightforward enough to calculate. It doesn't appear to interact through the strong force, so it can't make nuclei. It doesn't interact through the weak force, so its particles don't decay into lighter ones. It doesn't interact through the electromagnetic force, so it doesn't make molecules, nor does it absorb, emit, reflect, or otherwise scatter light. It exerts gravity that ordinary matter responds to. End of story. Beginning of ignorance.