Scientists have spotted the "rarest event ever recorded", in a major breakthrough as part of attempts to solve a dark matter mystery.

The process takes more than one trillion times the age of the universe. And it has been spotted by scientists for the first time.

The breakthrough discovery was made by scientists who worked with a special instrument, built to track down dark matter, the most elusive particle in the universe.

The all-important event was the radioactive decay of xenon-124.

"We actually saw this decay happen. It's the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it," said Ethan Brown, an assistant professor of physics at Rensselaer, and co-author of the study.

The most incredible space images of Earth Show all 30 1 /30 The most incredible space images of Earth The most incredible space images of Earth Striking Africa Explore ESA astronaut Tim Peake's stunning photos of Earth, taken from the International Space Station during his six month mission (captions by Tom Peake) "The striking colour and texture of Africa Illizi, Algeria" The most incredible space images of Earth Favourite Reef "Every day spent living in space is a great day, but today was particularly special. I got to speak with one of my inspirational heroes Prof Stephen Hawking and his amazing daughter Lucy, who developed the Principia Space Diary to engage children with STEM subjects. As well as talking about dark matter, quantum entanglement, alien life and light beam powered nanocraft we also got to see an amazing pass over the Bahamas and this - my favourite reef smile emoticon" The most incredible space images of Earth Russia's north-east coast "Sunrise approaching Russia's frozen north-east coast" The most incredible space images of Earth Hello London "Hello London! Fancy a run? :) #LondonMarathon" The most incredible space images of Earth Bahamas "50 shades of blue: Bahamas" The most incredible space images of Earth Yinchuan "Snow on the mountains next to Yinchuan in China" The most incredible space images of Earth Rocket flames in Africa "Is it just me or do I see some rocket flames down there? These strange land features are in the Erg Iguidi desert, with its yellow stripes of sand stretching from Algeria to northern Mauritania in the Sahara" The most incredible space images of Earth Stunning colours "Sunlight reflecting the stunning colours of this Himalayan lake" The most incredible space images of Earth The real Everest "The real thing: found Everest! Last picture turned out to be third-tallest mountain Kanchengjunga" The most incredible space images of Earth Go Exomars "Go #Exomars – have a great mission. Earth has more in common with Mars than you might think… #AfricaArt" The most incredible space images of Earth Tenerife "Amazingly clear view of Tenerife" The most incredible space images of Earth Midday winter sun "Some midday winter sun glinting off Greenland’s snow-capped peaks" The most incredible space images of Earth Sand dunes "Great texture in these huge sand dunes, Saudi Arabia" The most incredible space images of Earth Dragon Dam "The dam makes this river look like a dragon’s tail. Oahe Dam north of Pierre, South Dakota in the United States. (North is to the right)" The most incredible space images of Earth Smoking volcano "Spotted volcano smoking away on Russia’s far east coast this morning – heat has melted snow around top" The most incredible space images of Earth New Zealand "New Zealand looking stunning in the sunshine. Mt Cook centre left with the Grand Plateau to the front and Mt Tasman (3,497m) to the right of the Grand Plateau. Fox Glacier in the middle then Franz Josef curving right. Tasman Lake (largest at front) is at the foot of the Tasman glacier which runs along the front of them. The Hooker Glacier flows out behind Mt Cook coming down to meet the Mueller Glacier on the left of the photo. The Murchison Glacier is at the front of the photo running parallel with the Tasman Glacier" The most incredible space images of Earth Plankton bloom "Another great pass over Patagonia and a swirling plankton bloom off the coast" The most incredible space images of Earth Alaska "We don’t often get such clear views of Alaska" The most incredible space images of Earth Lights along the Nile "Lights along the Nile stretching into the distance from Cairo" The most incredible space images of Earth Kamchatka "The Pacific ‘Ring of Fire’ clear to see amongst the volcanoes of Kamchatka, Russia" The most incredible space images of Earth Cumulonimbus "I’m guessing there was an impressive storm going on under that cumulonimbus cloud" The most incredible space images of Earth Night Sahara "Night-time Sahara – you can really see how thin the Earth’s atmosphere is in this picture" The most incredible space images of Earth Japan "Tokyo and Japanese coast. This image shows most of Japan with the largest mass of light corresponding to Tokyo. The white lights on the left are fishing boats" The most incredible space images of Earth Morning sun volcanoes "Morning sun striking active volcanoes in Guatemala" The most incredible space images of Earth Tapajos River "The vast waters of the Tapajos river, Amazonia" The most incredible space images of Earth Patagonia "Beautiful glacial river water flowing from this Patagonian ice field Lake Viedma, West is up" The most incredible space images of Earth Dubai Palms "Minus the #Dragon photobomb this time..." The most incredible space images of Earth Sediment in Ethiopia "Sediment spilling into this mountain lake, Ethiopia" The most incredible space images of Earth Italy "We have phases of ‘short nights’ on the International Space Station – sunlight is nearly always visible right now. No prizes for guessing where this is…" The most incredible space images of Earth Panama Canal "From one mighty ocean to another – ships passing through the Panama canal"

"It's an amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded."

The research came from the XENON Collaboration, which runs an instrument known as XENON1T. That is a 1,300-kilogram vat fille dwith super-pure liquid xenon, which can be shielded from cosmic rays by being buried in water, 1,500 meters beneath the Gran Sasso mountains in Italy.

It is run by scientists looking for dark matter, which is five times more abundant than ordinary matter, but has never been directly observed. They hope to study it by watching for the little flashes of light that are created when particles hit the xenon inside the detector.

Though it is looking for the interaction between dark matter particles and the nucleus inside the xenon atoms, it can actually see any signals that happen inside the tub.

It was one of those interactions that scientists spotted, when they saw a proton inside the nucleus of a xenon atom change into a neutron. That happens as a consequence of a very rare event: when a proton absorbs two electrons, which is called "double-electron capture".

That can only happen when two of those electrons are right next to the nucleus, at exactly the right time. That is "a rare thing multiplied by another rare thing, making it ultra-rare", Brown said.

Scientists spotted those electrons re-arranging as they made way for the two electrons that had been absorbed into the nucleus.

"Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell," said Brown. "The remaining electrons collapse to the ground state, and we saw this collapse process in our detector."

The discovery allows scientists to see the half-life of the xenon isotope directly, watching its radioactive decay as it happens. Such an achievement has never been possible before, the researchers said.