When we look at the night sky, darkness seems to fill the spaces between the stars.
But what if some of the most powerful objects in nearby galaxies are hiding inside that darkness – not because they are distant or faint, but because they shine in light we can barely detect?
Astronomers have now found 84 of them.
The mysterious objects had been sitting unnoticed in observations from NASA's Chandra X-ray Observatory.
Some may release hundreds of thousands to millions of times more energy than the Sun, yet nobody had recognized them as a distinct group.
It is like discovering 84 powerful lamps in thick fog by noticing only the faintest glow escaping through it.
A new study has now brought this hidden population into view.
"What surprised us most is that, despite being so difficult to detect, they can be extremely luminous," University of Alabama astronomer Mustafa Muhibullah told ScienceAlert.
The researchers named them hypersoft X-ray sources. Here, "soft" simply means their X-rays carry relatively little energy.
Most of their radiation probably emerges as extreme ultraviolet light, or EUV, which lies between ultraviolet light and X-rays.
Gas between the stars absorbs EUV light, preventing it from reaching our telescopes. Chandra detects only the small amount of low-energy X-ray light that escapes.
"What Chandra sees is only the faint, low-energy X-ray tail of that emission," Muhibullah says.
The team found between 7 and 21 sources in each of six galaxies, including Andromeda and M101.
But how do we know these faint points aren't simply errors in the telescope's detector?
The first clue was repetition: many appeared at exactly the same positions in observations made at different times. A random error would not keep returning to the same place. Some were also detected by another telescope, XMM-Newton.
Their locations offered a second clue.
In M101, many followed its spiral arms. Others appeared among older stars in elliptical galaxies. Fewer than 3 percent are expected to be unrelated objects.
And 84 may be only the beginning.
Chandra collects little light at these energies, and contamination on its detectors has reduced this ability. The team may have found only the brightest members of a much larger population.
The next mystery is what these objects actually are.
There are several suspects. Some may contain white dwarfs: the small, dense cores left when stars like the Sun die. Others may have recently erupted as novae. Still others could contain neutron stars or black holes feeding on matter.
'Hypersoft' may not describe one kind of object. It describes the unusual light reaching the telescope. Several different cosmic systems may produce it.
Finding out what they are could solve another cosmic mystery.
When a star like the Sun dies, it can leave behind a white dwarf: an Earth-sized core packed with an enormous amount of matter.
If the white dwarf has a companion star, it can pull gas from it. That stolen gas collects on the white dwarf, making it hotter and heavier. In some cases, the white dwarf eventually destroys itself in a gigantic explosion called a Type Ia supernova.
These explosions help astronomers measure cosmic distances. Yet there is a strange gap in the story: they see plenty of explosions, but not enough white dwarfs preparing to explode.
It is like arriving after a fire and finding the wreckage, but not knowing what was burning before it began.
The 84 sources could include some of those missing stars. If certain hypersoft sources are white dwarfs quietly collecting gas, astronomers may have caught them during the long build-up before an explosion. They went unnoticed because most of their energy was hidden in EUV light.
"If even a fraction of this large – and probably much larger hidden – hypersoft population consists of accreting, nuclear-burning white dwarfs, they could help provide part of the missing observational link," Muhibullah says.
This does not mean all 84 will explode. The group may also include neutron stars, black holes, and white dwarfs that never become supernovae. But identifying even a few future Type Ia supernovae would give astronomers a rare look at what happens before the blast.
The sources may also affect their surroundings. Their powerful EUV light can change gas inside galaxies, meaning a hidden population could be quietly shaping its cosmic neighborhood.
To unmask them, astronomers need better temperature measurements. Observing the sources in ultraviolet and X-ray light at nearly the same time could reveal which suspect lies behind each glow.
Related: First Visual Evidence Confirms A Star Exploded Twice
For now, the 84 objects show that even powerful telescopes have blind spots – and that entire populations may be shining just beyond the limits of our sight.
The study was published in Nature Astronomy.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.