Bit by bit, physicists are winnowing down the potential masses for hypothetical particles called axions.

If they exist, the subatomic particles could make up dark matter, a mysterious source of mass that pervades the universe. Axions are expected to be extremely lightweight — billionths or trillionths the mass of an electron. But there were no sightings of the elusive particles in a mass range between 2.81 millionths and 3.31 millionths of an electron volt (between about 5.5 trillionths and 6.5 trillionths of an electron’s mass), physicists with the ADMX experiment report in a paper in press in Physical Review Letters.

Scientists expect axions to have masses between a millionth and a thousandth of an electron volt. Previously, ADMX searched a small range of masses, and scientists are now expanding that range (SN: 4/9/18). The new result is “one step on a long road to exploring the whole plausible range,” says physicist Gray Rybka of the University of Washington in Seattle, co-spokesperson of ADMX.

Another team of physicists searched for axions with masses around 6.7 millionths of an electron volt, according to another paper in press in Physical Review Letters. Those researchers, part of the CAPP-8TB experiment, likewise found no signs of the particles, although their experiment is not yet sensitive enough to exclude the possibility that axions might be present in that mass range.

Both experiments search for signals produced when axions convert into particles of light, photons, in a magnetic field. The detectors are tuned to search for certain frequencies of light, which correspond to different masses of axions.

Most previous dark matter experiments have searched for a different type of hypothetical particle called a WIMP, but have turned up nothing, leading some to pin their hopes for finding dark matter on axions (SN: 10/25/16).

With each search of a new mass range, there’s a chance for spotting axions, says Rybka. “We could get lucky.”