Astronomers are not too clear on the biggest question in the universe: "what the heck is all this?"
We know some of the Universe is made up of matter. This includes everything we can detect as existing in the world -- humans, other animals, the Earth, amoebas, microbes, the billions of stars and galaxies in space. Everything.
But here's the shocker: "everything" doesn't cover everything in the Universe. Not even close.
The first one to notice it was Fritz Zwicky. It was 1933. The Swiss astronomer was studying the Coma Cluster of galaxies, which are 320 million light years from Earth. Or 1.9 sextillion miles. (That's a ten with twenty-one zeros after it -- very far away.) Zwicky calculated how fast the galaxies were moving through space. The answer? 1,200 miles a second, or 4 million miles an hour. His answer was correct, but it was also impossible. The cluster of galaxies was holding together as they moved through space, but they shouldn't be. At those extreme speeds, they should have flown apart. If the only thing holding them together was gravity from the galaxies themselves, they would not have held together at all.
There must be something else.
Zwicky proposed there was invisible matter, enough to generate a gravitational hold on the clusters. He called it "dunkle materie". Or, in English, dark matter.
We've done a lot of math since 1933. Turns out, by measuring how galaxies move, how light bends through space, and how cosmic microwaves form patterns, we've been able to determine how much matter is in the Universe, and how much dark matter must be out there to explain how everything stays together.
Bottom line, only 5% of the Universe is "everything" we see. A mind-numbing 95% is invisible, either dark matter or dark energy. We know it's there because it clearly exerts forces on us and all the galaxies and stars around us.
No one has ever detected dark matter, not directly. We keep trying. There have been numerous, elaborate experiments. The world's biggest is the LUX-ZEPLIN built a mile underground at the Sanford Research Facility in South Dakota. It cost nearly $150 million to build. It uses ten tons of ultra-pure liquid xenon and looks for flashes of light or electrical signals when dark matter collides with a xenon nucleus. The facility collects data continuously and will run for years more. Scientists estimate the kind of event LUX-ZEPLIN is looking for is extremely rare - it may only happen a few times a year.
So far, it hasn't happened at all. No dark matter has been detected. Not with LZ, not with any of the numerous other efforts. Not with anything.
We live with a mystery force around us we can't see. Where is Ben Kenobi when we need him?
Regardless of whether we detect dark matter, this theory of the universe's structure has been widely accepted for the last 25 years. The dark matter must be there. The math just works.
Although, there was a missing piece.
Based on the Dark Matter model, there should be "orphan" or "ghost" galaxies all around our enormous Milky Way. These smaller galaxies would have had their stars partially stripped away over billions of years by the Milky Way's gravity. This would leave behind faint remnants of the former galaxies that would orbit in the Milky Way's outer halo. But we have not found these expected orphan galaxies, not in the numbers that we expect. That is, if this dark matter theory was true.
This year, cosmologists at Durham University in the UK took a second look for these ghost galaxies. And this time they used a new technique, combining the highest-resolution supercomputer simulations that exist with novel mathematical modeling.
They are presenting their ongoing research at the Royal Astronomical Society's National Astronomy meeting this month.
Their findings? The ghosts are out there. The Durham models show as many as 100 more satellite galaxies surrounding the Milky Way, orbiting at close distances.
These ghost galaxies weren't ripped apart as they sped through space. They weren't broken up by the power of the Milky Way's gravity. Instead, they've held together for billions of years.
By what force? We really don't know. But we know it's there.
We know it's everywhere.