Galaxies spin faster than they should. Space is spreading apart when it shouldn't. And it's all starting to feel like we're forever going to be in the dark when it comes to the big questions in physics.
One physicist's solution lies in a hypothetical 'fluid' with negative mass. No, such a material has never been seen before. But the hunt for exotic particles and energies is getting harder by the day, so it's worth keeping our options open.
Jamie Farnes from the University of Oxford suggests we go back to Einstein's theory of general relativity – the one that describes gravity in terms of space-time geometry – and tweak it a little so it permits matter with negative mass to pop into existence.
This emergence of a strange 'pushing' particle could solve two of physics' most frustrating mysteries – why do galaxies hold together as they spin? And why does the Universe seem to be growing so quickly today compared with the past?
Right now, the best explanation for each observation is that very hard-to-see stuff is doing a lot of pushing or pulling.
Dark matter is whatever happens to be responsible for 'pulling' together stars and galaxies in addition to everything we can see. It's more than likely some kind of massive particle that doesn't interact well with visible matter, making it virtually invisible.
Dark energy, on the other hand, is a theoretical phenomenon responsible for counteracting gravitational forces, causing large scale structures to move apart and making the Universe look like it's expanding at an ever increasing rate.
Right now, they're the best answers we've got. While there are plenty of suggestions as to what lies behind each of them, we're still little closer to a smoking gun. That's in spite of the fact the two combine to make up roughly 95 percent of all the energy and matter in the cosmos.
"It's embarrassing," Farnes says in his article on The Conversation. "But astrophysicists are the first to admit it."
Farnes wonders if this dark 95 percent all comes down to the same thing. He's proposed an all-permeating dark 'fluid' that appears in empty space and weakly pushes against surrounding matter.
Not only would this gentle nudge drive galaxies apart, creating additional space for more dark fluid to 'pop' into reality, it would push in on their stars, stopping them from slipping free as the galaxy whirls.
As far as potential theories go, it feels rather parsimonious. Nothing quite like a two-for-the-price-of-one solution.
And better still, Farnes' negative mass models could be put to the test using data on the distribution of galaxies collected using the Square Kilometre Array.
"The outcome seems rather beautiful," says Farnes.
"Dark energy and dark matter can be unified into a single substance, with both effects being simply explainable as positive mass matter surfing on a sea of negative masses."
Beautiful, sure. But even Farnes agrees the idea is a little out there as far as the supporting physics goes.
First of all, while there are phenomena that exhibit negative mass-like characteristics, they're not the same thing as spontaneously appearing negative mass particles.
Secondly – while quantum mechanics does predict particles popping in and out of existence in a vacuum – this also doesn't add up to the perpetual generation of a dark soup of negative masses.
Still, before we get ahead of ourselves, Albert Einstein himself proposed a similar fudge factor as he sketched out general relativity. So there's room in the mathematics to account for such a concept.
"In terms of the Newtonian theory," he wrote back in 1918, "modification of the theory is required such that 'empty space' takes the role of gravitating negative masses which are distributed all over the interstellar space."
Mind you, he also dismissed the negative mass of empty space as his biggest blunder.
Still, whatever appears to be pushing space apart and holding galaxies together, we don't have an answer.
We could use a few more suggestions up our sleeve in case all other ideas fail. In which case, Farnes' outlandish model of a universe leaking dark fluid just might see its day in the Sun after all.
"If real, it would suggest that the missing 95 percent of the cosmos had an aesthetic solution," he says.
"We had forgotten to include a simple minus sign."
This research was published in Astronomy & Astrophysics.