In the heart of a galaxy cluster 200 million light-years away, astronomers have failed to detect hypothetical particles called axions.

This places new constraints on how we believe these particles work - but it also has pretty major implications for string theory, and the development of a Theory of Everything that describes how the physical Universe works.

"Until recently I had no idea just how much X-ray astronomers bring to the table when it comes to string theory, but we could play a major role," said astrophysicist Christopher Reynolds of the University of Cambridge in the UK.

When it comes to understanding how the Universe works, we've developed some pretty good frameworks. One is general relativity, describing how physics works on a macro level. Another is quantum mechanics, which describes how things behave on the atomic and subatomic level.

The big problem is that the two frameworks famously don't get along. General relativity cannot be scaled down to the quantum level, and quantum mechanics cannot be scaled up. There have been many attempts to get them to play nice, developing what is called a Theory of Everything.

One of the most promising candidates for resolving the differences between general relativity and quantum mechanics is something called string theory, which involves replacing the point-like particles in particle physics with tiny, vibrating one-dimensional strings.

Furthermore, many models of string theory predict the existence of axions - the ultra-low-mass particles first hypothesised in the 1970s to resolve a question of why strong atomic forces follow something called charge-parity symmetry, when most models say they don't need to. As it turned out, string theory also predicts large numbers of particles that behave like axions, called axion-like particles.

One of the properties of axion-like particles is that they can convert into a photon when they pass through a magnetic field; and, conversely, photons can convert into axion-like particles when they pass through a magnetic field. The probability of this depends on a range of factors, including the strength of the magnetic field, the distance travelled, and the mass of the particle.

This is where Reynolds and his team come in. They had been using the Chandra X-ray Observatory to study the active nucleus of a galaxy called NGC 1275 that sits around 237 million light-years away, at the heart of a cluster of galaxies called the Perseus cluster.

Their eight days' worth of observations ended up telling them almost nothing about the black hole. But then they realised the data could be used to look for axion-like particles.

"The X-ray light from NGC1275 needs to pass through the hot gas of the Perseus cluster, and this gas is magnetised," Reynolds explained.

"The magnetic field is relatively weak (more than 10,000 times weaker than the magnetic field at Earth's surface), but the X-ray photons need to travel an enormous distance through this magnetic field. This means there is ample opportunity for the conversion of these photons into axion-like particles (provided that the axion-like particles are sufficiently low mass)."

Because the probability of conversion depends on the wavelength of the X-ray photons, the observations should reveal a distortion as some wavelengths are being converted more effectively than others. It took the team about a year of painstaking work, but in the end, no such distortion was found.

This means the team could rule out the existence of axions in the mass range their observations were sensitive to - down to about a millionth of a billionth of the mass of an electron.

"Our research doesn't rule out the existence of these particles, but it definitely doesn't help their case," said astronomer Helen Russell of the University of Nottingham in the UK.

"These constraints dig into the range of properties suggested by string theory, and may help string theorists weed their theories."

The research has been published in The Astrophysical Journal.