The LCLS-II X-ray laser in California fires short, powerful flashes, like a strobe light at a disco. Between flashes, there are dark pauses lasting 26.9 nanoseconds (billionths of a second). It is precisely in these pauses that physicists want to catch dark matter — the mysterious substance that doesn't emit light but pulls on ordinary matter with its gravity.
The idea is this: during the pause between laser flashes, send a single electron into a thin target. If dark matter exists as light particles, sometimes an electron might collide with one and transfer some of its energy. Then, after the target, a detector would see that the electron lost energy even though nothing visible was created. Such a "disappearance" would be the signal of dark matter.
But there's a complication: even during the pauses between flashes, stray electrons, so-called dark current, can fly through the accelerator. This creates background that could spoil the measurement. The scientists built a prototype detector from scintillators — plates that flash with light when a particle passes through. They placed it in the beam path and counted how many stray electrons arrive during the pauses. It turned out that there are very few — millions of times fewer than needed for a useful signal. This is good news: the empty gaps of the laser are indeed clean, and we can search for dark matter in them.
So the pauses between laser flashes are not empty time but a quiet corridor where we can hear the whisper of dark matter. If the experiment succeeds, we will catch for the first time a particle that makes up most of the matter in the universe.
The distance between electron bunches in LCLS-II is 26.9 nanoseconds. In that time, light travels only 8 meters, while an electron in the accelerator travels less than a centimeter.