They are looking for dark photons:
When positrons slam into the diamond wafer, they immediately merge with electrons and vanish in a faint burst of energy. Normally, the energy released is in the form of two particles of light called photons. But if a fifth force exists in nature, something different will happen. Instead of producing two visible photons, the collisions will occasionally release only one, alongside a so-called “dark photon”. This curious, hypothetical particle is the dark sector’s equivalent of a particle of light. It carries the equivalent of a dark electromagnetic force.
Unlike normal particles of light, any dark photons produced in Padme will be invisible to the instrument’s detector. But by comparing the energy and direction of the positrons fired in, with whatever comes out, scientists can tell if an invisible particle has been created and work out its mass. Though normal photons are massless, dark photons are not, and Padme will search for those up to 50 times heavier than an electron.
The dark photon, if it exists, would have an imperceptible influence on what makes up the world we see. Ian Sample, “Scientists hunt mysterious ‘dark force’ to explain hidden realm of the cosmos” at The Guardian
The “dark photon” is already having an “invisible influence on the world we see,” to judge from the language used to describe the project above: a project of fanciful but seemingly hopeless desperation Sabine Hossenfelder explains it fairly well: Particle physics now belly up.
See also: Biggest mystery cosmology may not exist says top physicist 2014? Given the importance attached to dark energy, it’s surprising that these objections were not raised sooner.
and
Particle physicist: Science is suffering from “baked in” bias The challenge is simpler than sometimes supposed. People must be willing to accept a truth they don’t like.