Alex McColgan

There, bound up as vast diffuse gas in those long strings or filaments, is where large-scale cosmological simulations have predicted we might not only find dark matter, but our missing visible matter too.

This low-density, diffuse gas is known as the warm-hot intergalactic medium.

But simulations are one thing, detecting these diffuse filaments is another.

And this is where historically, we've not had much luck, although that started to change about 20 years ago.

In 2005, NASA's Chandra X-ray Observatory imaged two huge intergalactic clouds of diffuse gas.

We'd seen clouds like this around our own galaxy and in others local to us, but not between distant galaxies.

Could it be gas on the cosmic web?

This was some of the first evidence that the cosmic web was hiding our missing mass, but the light was too faint to completely isolate it, so scientists couldn't quite be sure.

Then, in 2012, a combination of 18 Hubble images was used to infer the presence of a filament funnelling matter into the galaxy cluster Max J0717.

Scientists found it by studying the distortion of light from background galaxies due to the gravity of the filament's dark matter.

But again, it wasn't a direct detection.

We still couldn't say we've actually seen them.

Inching ever closer to a discovery, it was just two years later that astronomers moved beyond indirect detection and statistical evidence into the realm of the visible.

A team led by Sebastiano Cantalupo from the University of California observed an enormous filament of hydrogen gas.

At nearly 2 million light years across, they spotted it after it was illuminated by a giant cosmic flashlight, a bright quasar shining at it from

10 billion light years away.

This was the first time part of the cosmic web had been seen directly.

Using the same illumination principle, in 2019, astronomers revealed whole networks of hydrogen filaments surrounding a massive protocluster, directly imaging the web on megaparsec scales.

But these detections were only possible thanks to our cosmic flashlights.

To understand cosmic filaments as they typically exist, in darkness, we needed a new imaging technique.