Ever since the late 1990s, physicists have been fairly certain that the Universe isn't only getting bigger, it also appears to be expanding at an ever increasing rate.

A mysterious force called dark energy is currently thought to be responsible for this accelerating growth, but a new study raises the possibility that what seems to be a type of energy could be an illusion caused by the changing structure of the Universe.

Physicists from Loránd University in Hungary and the Institute for Astronomy at the University of Hawaii are now questioning if approximations in Einstein's equations introduced "serious side effects" that gave the illusion of a vast, unknown force pushing space apart.

If it exists, dark energy would make up about 68 percent of the energy in the observable Universe, but at just 10-27 kilograms per square metre, it would be incredibly hard to spot in the laboratory.

In addition to the question of acceleration, dark energy also helps explain things like the overall shape of the Universe and the patterns of matter we see rippling through space.

The thing is, right now it's little more than an empty box without any other properties to describe the nature of its existence.

As such, it's currently assumed to be a fundamental part of empty space known as the cosmological constant, represented by the Greek letter lambda (Λ).  

Back in the early 20th century, Einstein proposed the cosmological constant as a kind of fudge-factor to explain why all the mass scattered through the Universe wasn't pulling back together under the attraction of its own gravity.

When Edwin Hubble made it clear the Universe wasn't just resisting collapse, but actually expanding, the cosmological constant was thrown in the bin.

It's now known that the Universe appeared to grow at a slower rate in its youth than today, making the cosmological constant useful again as a way to explain this increase in speed.

Put together with another hypothetical 'black box' factor – dark matter, which would comprise of a further 27 percent of the known Universe – we get the Lambda Cold Dark Matter (ΛCDM) model to explain how the Universe evolved.

While Einstein's general theory of relativity was responsible for laying much of the groundwork for this model, the mathematics isn't always so easy to apply, prompting physicists to crunch parts of it down using educated assumptions.

But in this latest study, the researchers argue these approximations have ignored potentially significant influences of large scale structures within the Universe.

"Einstein's equations of general relativity that describe the expansion of the Universe are so complex mathematically that for a hundred years no solutions accounting for the effect of cosmic structures have been found," said László Dobos from Eötvös Loránd University.

If it were possible to step outside of the Universe for a moment and look down upon it, there'd be threads of galaxies called super clusters lining what look like relatively empty spaces.

The ΛCDM model assumes a uniform expansion that progressively gets faster thanks to the increasing push of dark energy overcoming the pull of dark and normal matter distributed evenly throughout space.

Yet according to the physicists involved in this new research, the large scale structures – 'bubbles' of seemingly empty space and the galaxies surrounding them – would create zones where expansion occurs at different rates, almost like mini-universes.

By mathematically modelling the effect of gravity on millions of particles representing dark matter, the team managed to recreate the bunching up of matter in the early Universe in such a way that it looked like the large scale galaxy structures.

While the Universe in their model still expands, the individual differences in how these bubbles expand averages out to an overall acceleration.

"Our findings rely on a mathematical conjecture which permits the differential expansion of space, consistent with general relativity, and they show how the formation of complex structures of matter affects the expansion," said Dobos.

"These issues were previously swept under the rug but taking them into account can explain the acceleration without the need for dark energy." 

The model makes its own necessary assumptions, but if it stands up to scrutiny it could explain why the Universe's expansion seems to be accelerating, all without the need for negative pressure.

While the idea itself is new, the search for ways to get around the need for a mysterious type of energy has produced a number of creative solutions in recent years.

Earlier this year, a study published in Science suggested dark energy could be explained as a kind of deficit, as if the Universe was 'leaking energy' at some point in its evolution.

While it breaks one of the big rules of physics (energy can't be lost or created) it would also take care of the nagging question mark over what 68 percent of the Universe is made out of.

There's no doubting dark energy is a tough nut to crack, so it might take thinking outside the box – if not outside the whole Universe – to find a solution.

This research was published in Monthly Notices of the Royal Astronomical Society.