There was some unknown entity, huge and invisible, filling the shadowy spaces between the stars – a whole new type of matter.

Professor Ken Freeman, one of Australia’s most influential astronomers and recent recipient of the nation’s most prestigious science prize, had absolutely no interest in the field when he started out. The world-renowned  galaxy- gazer says he “got into it almost by chance”.

“There was no special moment the first time I looked through a telescope,” he says. “I’ve never been into that side of astronomy – the thrill of the sky and all that stuff. It’s all about the physics for me.”

Freeman was completing an undergraduate degree in mathematics in Perth when he spotted a flyer on a noticeboard advertising the Mount Stromlo Observatory student vacation scheme.

“I was really just looking for something I could use my mathematics degree for,” he says. “I considered all sorts of things – even actuarial studies! But that flyer caught my attention.”

The young Freeman applied for the program and received a very enthusiastic reply from then Director, Bart Bok.

“Everybody in Canberra knew about Bart Bok,”recalls Freeman. “He was a real character.

“As a promoter of astronomy he was absolutely unparalleled. He wrote a very enthusiastic letter back to me about all the exciting things they were doing and said that I should come along.”

That letter sparked the beginning of a meteoric 50-year career. Freeman attended the vacation program twice and, after completing a PhD at the University of Cambridge (“Stephen Hawking started in the same department the same year that I did”) and a post-doc in Texas, he returned to ANU in 1967 and has been here ever since.

It was in his first few years at ANU that Freeman made the discovery that would see him celebrated by the scientific community, honoured by the Prime Minister and dubbed ‘Mr Galaxy’ by ANU Nobel Laureate and fellow astronomer Professor Brian Schmidt.

Freeman was calculating the rotation speed of spiral galaxies like our own Milky Way. When he compared the actual rotation speeds with what the maths showed they ought to be, he discovered that the galaxies were spinning much faster than would be predicted on the basis of the mass of the stars inside them. There was some unknown entity, huge and invisible, filling the shadowy spaces between the stars – a whole new type of matter.

“What you see when you look at a spiral galaxy –the typical flat, spinning disc like a Catherine wheel – is only a tiny fraction of its total mass,” explains Freeman. “We now know that about 97 per cent of the total mass of these galaxies is invisible dark matter.

“And it turned out around 84 per cent of the matter in the entire Universe is dark matter. At this point we don’t know what it is. It’s one of the biggest problems in astrophysics today.”

Surprisingly, it wasn’t the dark matter discovery that got people excited when Freeman published the findings in 1970.

“That paper actually had another discovery in it, which, incidentally,  is now called Freeman’s Law,” he explains. “It was a very unexpected effect – that the brightness of the surface of disc galaxies is always the same, no matter what size they are. I remember the night I discovered that very clearly.

“I used to come back into the office after dinner to work and that night a very famous visitor called Allan Sandage was also working late. I brought the data in to him and I said, ‘Hey Allan, I’ve found something interesting’. And I remember him looking at this stuff and just putting his hands over his face. He couldn’t believe it – it was just too much.

“So when we published the paper, there was a lot of interest in this other bit, which Allan had got so excited about, but there was almost no reaction at the time to the dark matter aspect of it. That didn’t start rolling until later.”

Although the existence of dark matter is now taken as reality, the idea was initially met with scepticism, even by Freeman himself. When asked if he was convinced by his original finding, he replies: “Oh, absolutely not! The data was really crummy. The equipment back then was nothing like what we have now.

“Until we got these radiotelescopes that could do very detailed work, the subject of dark matter was very uncertain. The argument about whether or not dark matter existed went on for about a decade, but it was really just because of the data quality.”

In his 45 years at Mount Stromlo, part of the ANU College of Physical and Mathematical Sciences, Freeman has witnessed many changes; he’s seen many directors come and go since the charismatic Bart Bok and turned down more job offers than he can remember. But the most devastating event was the bushfire of 2003, in which Freeman and his wife, along with 500 others, lost their family home. While Freeman’s office of more than 30 years survived mostly intact (“a huge fireball roared over the building and cracked the windows and shrivelled the curtains, but only a few papers on mydesk were singed”), most of the other buildings and all the telescopes were destroyed.

“We had the fire on Saturday, and by Monday our computer guys had us all set up in little cubicles on the Acton campus,” Freeman recalls. “Meanwhile Facilities and Services got the windows reglazed and the water and electricity back on, and in three weeks we were back on Mount Stromlo. It was an amazing and fantastic effort by the staff, and we are immensely grateful for what they were able to do for us.”

One thing has stayed constant during Freeman’s time at Stromlo: the vacation program he first attended in 1959 is still running.  Freeman is himself an enthusiastic and committed supervisor and has guided more than 50 PhD students in their own journeys of discovery.

And there are plenty of discoveries to come. At 72 and with no plans to retire, Freeman is eager to get started on the latest project in his stellar career: analysing the chemistry of one million star ‘fossils’  to trace the assembly history of our galaxy over its ten-billion-year existence. Not bad for a man who happened upon astronomy by chance.