A faint and ghostly galaxy 10 million light-years away has delivered one of the holy grails of black hole astronomy.

At its heart, lies a black hole that looks to belong to an elusive middleweight class of intermediate-mass black holes, a discovery that could help us understand how some of the most massive black holes form.

The galaxy in question is a dwarf galaxy called Mirach's Ghost (or, less poetically, NGC 404), and it's long been suspected to harbour one of these 'missing link' intermediate-mass black holes. Now, a new technique seems to have validated this suspicion, with scientists discovering a black hole inside Mirach's Ghost with a mass 550,000 times that of the Sun.

While the boundaries between intermediate-mass black holes (IMBHs) and supermassive black holes (SMBHs) are currently not very well defined, IMBHs are generally considered to be larger than a typical collapsed star (up to a hundred solar masses) but not supermassive (between a million and a billion times more mass than a typical stellar black hole).

So, the new discovery's seemingly middleweight mass makes it an important object for understanding how supermassive black holes form and grow.

Supermassive black holes are a huge conundrum. We have a pretty good handle on how the smaller, stellar-mass black holes form - they are the dead, collapsed cores of massive stars, and can be up to a few tens of solar masses.

But there's an upper limit to this formation model imposed by the mass of the precursor star. If the star starts out with a mass between 130 and 250 solar masses, it ends its life in what is called a pair-instability supernova that blows the star apart.

You may have noticed that there's a big gap between stellar mass black holes and supermassive black holes. That's where intermediate-mass black holes should fall, but they've proven incredibly difficult to actually find.

This poses a problem, because if black holes start from little wee itty stellar-mass ones, as proposed by one evolution model, and grow into mighty beasts by accreting lots and lots of matter over a very long time, then intermediate-mass black holes would logically be the step between the two.

The other possibility is that supermassive black holes were just born that way, directly collapsing from a huge clump of matter already in the nuclei of galaxies. And supermassive black holes have been found in the early Universe, way too soon after the Big Bang to have had the time to grow from stellar mass black holes.

If this were the case, though, there would be a lower limit on the mass of supermassive black holes.

One way to learn more would be to discover intermediate-mass black holes. They wouldn't necessarily invalidate the direct collapse model, but they would be a big tick in the favour of the accretion model.

We've actually had some pretty convincing indirect observations that suggest the existence of these middleweights - but astronomers believe that even more solid evidence can be found in the nuclei of small galaxies, aka dwarf galaxies.

Dwarf galaxies tend to preserve clues on their history of black hole evolution far better than larger, more battle-scarred relatives. Understanding the characteristics of their intermediate-mass black holes would be a big win for understanding how they grow.

Cue Mirach's Ghost, so-named because it's very hard to see, obscured by a much closer and very bright star. A decade ago, astronomers found evidence that a black hole of at least a few tens of thousands of solar masses was in its centre - but because the galaxy is hard to see, it was very difficult to learn more.

Two things have happened since then. The Atacama Large Millimeter/submillimeter Array (ALMA) in Chile - a state-of-the-art telescope with incredible resolution - came online in 2011. Then, in 2014, astrophysicists validated a technique to derive the mass of a black hole based on the movements of the gas around it.

This is what a team of astronomers led by Tim Davis of Cardiff University did. They used ALMA to observe Mirach's Ghost in high resolution, mapping the movement of gas around its core to a very high resolution of 1.5 light-years across.

Then they used simulation software to predict gas distribution and kinematics, comparing these results against the observations to obtain the best fit.

This is how they calculated the mass of the black hole. Given that the definition of 'intermediate-mass' isn't well defined, its classification in that category might be arguable. But, much more interestingly, it provides support for both supermassive black hole evolution models.

"The SMBH in Mirach's Ghost appears to have a mass within the range predicted by 'direct collapse' models," Davis said.

"We know it is currently active and swallowing gas, so some of the more extreme 'direct collapse' models that only make very massive SMBHs cannot be true. This on its own is not enough to definitively tell the difference between the 'seed' picture and 'direct collapse' - we need to understand the statistics for that - but this is a massive step in the right direction."

There have been other similarly low-mass supermassive black holes. A galaxy called NGC 4395 has a black hole calculated to be 360,000 solar masses, and the black hole at the heart of a galaxy called POX 52 was measured at 160,000 solar masses.

It's only by finding a whole lot more of these objects that astronomers will be able to start to put together the puzzle.

"Our study demonstrates that with this new technique we can really begin to explore both the properties and origins of these mysterious objects," Davis said.

"If there is a minimum mass for a supermassive black hole, we haven't found it yet."

The research has been published in the Monthly Notices of the Royal Astronomical Society.