Chapter 1: What is the universe's missing mass and why is it important?
Despite existing on a small planet in a tiny corner of the cosmos, astronomers know exactly how much visible matter the entire universe should contain. The problem is that for decades, 40% of it has been missing. And I'm not just talking about the ever mysterious dark matter and dark energy. Galaxy clusters don't seem to have as much visible mass as our models say should be there.
Entire galaxies seem to have lost huge reservoirs of the material they were born with. Even the space between galaxies, enormous cosmic deserts, are emptier than our best theories predict. This missing mass has to be out there. That, or all our models of cosmology, are wrong. But every time we've looked, we've found nothing. And believe me, we've tried.
We've used our most powerful telescopes, deepest surveys, and most sensitive detectors in the search. The missing mass just remains invisible, slipping past our instruments like a ghost. That is... until now. For the first time in history, we may have finally found where the missing mass of all the universe has been hiding, and it's in plain sight.
Astronomers have captured images of vast gaseous filaments that stretch 23 million light years between galaxy clusters. It may just be one filament, but finding it has huge ramifications. Is this the very first detailed image of the cosmic web? As scientists now hunt for more, this discovery has the power to determine whether our cosmological models are correct.
I'm Alex McColgan and you're watching Astrum. Join me as we reveal how astronomers have snatched a glimpse of the near-invisible network that underpins the cosmos, finally revealing the hiding place for our universe's missing visible matter. And with it, Let's start unraveling the truth behind our cosmic evolution.
When we measure all of the gas, dust, planets, stars and galaxies, everything we can see in the whole universe, using everything from infrared to visible light and beyond to gamma rays, it adds up to a colossal amount. More than 100 sextecillion kilograms. That's 10 with 53 zeros after it. But really, this only accounts for a small fraction of the total matter that scientists predict to exist.
In fact, this visible ordinary matter is thought to make up just 5% of the universe. The rest is stuff we don't completely understand. Dark matter is believed to account for 27%, and dark energy 68%. Despite their names, dark energy isn't related to dark matter. What they have in common is that we can't detect or see them.
Dark energy is thought to be a seemingly invisible type of energy, causing the universe's expansion to accelerate over time. And dark matter is a type of matter that has mass, but is invisible to us, as it doesn't absorb, reflect, or emit any light. I've talked about dark energy in previous videos, and that is truly its own mystery.
You can explore those if you're interested to find out more about the topic. But for this video, I'll just focus on the matter at hand. So, 95% of our universe is likely not made of visible, ordinary matter. And of the 5% that is technically visible matter, also known as baryonic matter, about 40% has been missing since the big bang 13.8 billion years ago. Now, I know what you're thinking.
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Chapter 2: How have astronomers attempted to locate the missing mass?
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Nearly a century ago, Swiss-born astronomer Fritz Zwicky noticed that galaxies in the Coma Cluster moved too quickly for the amount of gravity that would be created by their visible matter alone. Yes, including the missing stuff. Instead, he thought another form of mass must be there, and in 1933, Zwicky dubbed this missing substance Dunkelmaterie, the German for dark matter.
In the 1970s, American astronomer Vera Rubin had a similar experience. She was observing spiral galaxies and wondered how stars on the outer edge of the spiral galaxies were able to move so quickly without flying off into space. Again, some unknown mass must have been pulling them back in, and she concluded the same as Zwicky. There must be dark matter holding them together.
This new type of stuff interacts with its ordinary baryonic counterpart through gravity, but it doesn't interact with the electromagnetic spectrum, meaning that it doesn't absorb, reflect, or emit any light. This makes dark matter extremely difficult to find, and seemingly impossible to observe, at least directly. Yet, it influences the cosmos on a galactic scale.
it may even be helping to conceal our missing baryonic matter. Since the 1980s, Early observations of galaxy distribution have revealed a cosmic pattern, a web, like a backbone across the universe. This skeleton-like structure has become known as the cosmic web.
It's thought to have formed as a result of slight density fluctuations in the early universe, which can be seen in the Cosmic Microwave Background, or CMB. a fingerprint of our universe's ancient microwave radiation from about 380,000 years after the Big Bang. These tiny fluctuations in density laid out a blueprint for where matter would collect over space and time.
Where there were higher densities, more mass would be drawn in, until this great structure was formed. We already know that CMB simulations using supercomputers estimate that dark matter accounts for five times more of the universe than ordinary matter. so it's perhaps no surprise that the cosmic web is thought to contain primarily dark matter.
Permeating every corner of our universe, it's made of filaments stretching tens to hundreds of millions of light years across the universe. Where filaments intersect, the concentration of mass is so great that high density nodes are able to form, containing hundreds or even thousands of galaxies. The cosmic web is real. We've mapped large swaths of it.
Our galaxy is part of what's known as the Local Group, a collection of a few dozen neighbouring galaxies near to our own. Our Local Group belongs to a larger collection called the Laniakea Supercluster, which contains some 100,000 galaxies and measures 500 million light-years across.
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Chapter 3: What discoveries have been made about the cosmic web?
This was a spectacular achievement. But there was a catch. These observations were from the early universe. They traced cooler hydrogen gas billions of years in the past, not hot, diffused material thought to contain most of the universe's missing baryonic matter today.
Our models predict it stretched along thin filaments of the cosmic web in the local universe, but here the signal is far fainter and harder to isolate. We were stuck. In June 2025, astronomers finally found what they were looking for.
They were able to isolate and spectroscopically measure the hot, low-density gas of an individual cosmic web filament in the local universe, marking a breakthrough moment in our quest to locate the missing baryonic matter.
A team of European researchers headed by lead author Konstantinos Migas at Leiden University in the Netherlands used JAXA's Suzaku X-ray Space Telescope to map a single filament in faint X-ray emissions over a wide area of space.
They then used the XMM-Newton to pinpoint sources of X-ray contamination, in this case supermassive black holes, which had to be removed from the data in order to map the filament. In this image, you can see what they found. A filament of the cosmic web connecting four galaxy clusters, two on each end, each one as a white spot surrounded by colour.
The band of purple stretched between them, resembling a honeycomb, or bone marrow-like texture, is the filament of X-ray emitting hot gas. Located in the Shapley Supercluster, a supercluster of more than 8000 galaxies, the filament stretches across a distance of 23 million light years, which is the equivalent of about 230 Milky Ways end to end.
Its mass comes in at roughly 10 times that of the entire Milky Way galaxy, and the temperature of the filament's hot gas is a scorching 10 million degrees Celsius.
It reveals in detail for the first time how galaxy clusters are connected over colossal distances, and uncovers the vast cosmic web that underpins the structure of our entire universe, like the very bones of a cosmic skeleton upon which everything else forms.
And according to the paper's co-author, the filament is exactly what we expected from the best large-scale cosmological simulations of the universe. We got it right. The latest breakthroughs in observing filaments lend important evidence and support for our current standard model of cosmic evolution, known as the Lambda Cold Dark Matter model.
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