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Neutron stars are among the densest objects in the universe - just a spoonful of matter from one of them would weigh more than the moon.

But inside these remarkable stellar objects, which are no bigger than a city on Earth, a remarkable process might be taking place.

Scientists have revealed their matter might become so squashed that it turns into 'strange matter' - and observing so-called strange stars could unlock some of the secrets of the universe.

Scientists at the National Institute for Space Research in Brazil say an undiscovered type of matter could be found in neutron stars (illustration shown). Here matter is so dense that it could be 'squashed' into strange matter. This would create an entire 'strange star' - unlike anything we have seen

The latest theory was proposed by Dr Pedro Moraes and Dr Oswaldo Miranda, both of the National Institute for Space Research in Brazil.

They say that some types of neutron stars might be made of a new type of matter called strange matter.

What the properties of this matter would be, though, are unknown - but it would likely be a 'liquid' of several types of sub-atomic particles.

'Stars, galaxies, planets and even ourselves are made of the so-called baryonic matter,' Dr Moraes explained to MailOnline.

'A baryon, which is a proton or a neutron, is made up of more fundamental particles, known as quarks.

'There are six types (or flavours) of quarks. They are: up, down, strange, charm, top and bottom.

'Depending on how these quarks are combined, different baryons will be formed.

'For instance, two up quarks and one down quark form a proton while two down quarks and one up quark form a neutron.'

Inside a neutron star, however, Dr Moraes says the neutrons and protons of regular baryonic matter - the stuff that everything around us is made of - can be squeezed so intensely that it turns into a new type of matter - strange matter.

It would not be like any other type of matter we know - and would also be different to dark matter, as it could form physical objects.

Inside a neutron star, Dr Moraes says the neutrons and protons of regular baryonic matter (protons and neutrons illustrated) - the stuff that everything around us is made of - can be squeezed so intensely that it turns into a new type of matter - strange matter

The only known way to find strange matter at the moment would be to confirm its existence within neutron stars. On Earth, it is currently impossible to directly observe strange matter, even in places like the Large Hadron Collider at Cern in Switzerland. Pictured is the Large Hardon Collider Beauty experiment (LHCb)

'As its name says, a neutron star is a star made up of neutrons - which are made up of two down and one up quarks,' Dr Moraes continued.

'It is a star of very high density and rapid rotation rate. Most of them have masses close to 1.3-1.4 solar masses.'

Most matter we see comes in two 'flavours', made up of just two types of fundamental particles - up and down quarks.

But in these extreme conditions a rare type of three-flavour matter, made of up, down and strange quarks, could be being created.

This is what strange matter would be. And Dr Moraes says, if the neutron star is massive enough and rotating at a fast enough speed, the entire star could be made of this matter.

The star would be much smaller and lighter than a neutron star. For example, a neutron star with a mass 0.2 times that of the sun would have a radius greater than nine miles (15km), but a strange star of the same mass would be less than a third the size.

One of the implications of the theory, if true, would be that there might be more types of matter in the universe than we know of.

Dr Moraes says, as we cannot observe individual fundamental particles like quarks on Earth, the only way to prove strange matter's existence would be to spot it in a neutron star.

Interestingly, though, proving that strange stars exist could also provide a detection for one of the 'holy grails' of astronomy - gravitational waves.

Dr Moraes says the interaction of a neutron star and a strange star (illustration shown) could create ripples in space-times, resulting in gravitational waves. These are one of the 'holy grails' of astronomy that have been impossible to detect in other experiments so far

Gravitational waves are said to be ripples in space-time caused by the interaction of massive objects in the universe.

However, their existence has been hard to prove, despite numerous attempts to find them.

Dr Moraes says that, if strange stars exist, they could be interacting with regular neutron stars in binary systems and producing these noticeable effects.

Observing such a system could result in a 'gravitational wave detection,' said Dr Moraes, 'which is one of the most challenging experimental breakthroughs of the century.'

For now, just the existence of strange matter and stars themselves would be hugely interesting.

It would suggest there are objects in the universe that are far beyond our current understanding.