Dark matter's "nightmare scenario" looks more likely than ever

"There's an enormous puzzle to the Universe, and it's one that might be doomed to remain puzzling for a long time: dark matter. For generations, it has been recognized that the known law of gravity, Einstein's general relativity, combined with the matter and radiation that's known to exist in the Universe - including all the particles and antiparticles described by the Standard Model of physics - doesn't add up to describe what we see. Instead, on a variety of cosmic scales, from the insides of individual galaxies to groups and clusters of galaxies all the way up to the largest filamentary structures of all, an additional source of gravity is required."

"It's possible that we've got the law of gravity wrong, but if that's the problem, it's wrong in an extremely complicated way that also seems to require an additional source of matter (or something that behaves equivalently). Instead, the most common and successful hypothesis is that of dark matter: that there's an additional form of matter out there, and we feel its gravity, but have yet to experimentally detect it directly, the way we've detected all other confirmed particles."

"That hope, of direct experimental confirmation, is only possible if dark matter interacts with either itself or normal matter in a way that leaves a detectable signature. If dark matter's only interactions are gravitational, direct detection might truly be a physical impossibility for us. Unfortunately, that "nightmare scenario" might be exactly what's really happening here in our Universe, as all the evidence we have fails to turn up even a hint of an interaction beyond the purely gravitational. Here's what we're facing as this great nightmare starts looking more and more like our reality."

Observations from galaxy interiors to large-scale cosmic structure require more gravity than accounted for by known matter and radiation. Modified-gravity alternatives face complex constraints and often still imply matter-like contributions. The dominant explanation is an additional, non-luminous form of matter that produces gravitational effects but has not been detected through non-gravitational interactions. Direct experimental confirmation requires dark matter to interact beyond gravity, producing observable signatures. If dark matter interacts only gravitationally, direct detection could be physically impossible, and the absence of non-gravitational signals increasingly supports that challenging possibility.

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