Few people know what complete darkness is, Professor Charles Alcock announces to his audience.
The New Zealand-raised, world-leading astrophysicist tells a University of Auckland crowd that the concept of an absolute absence of light is a slippery one.
He shows a video of a 13-year-old girl who expects that her eyes will adjust to a darkroom "after four or five minutes". Her eyes never adjust, but she is comically certain that the darkness will lighten eventually.
"Most people have never been in a completely dark environment," Professor Alcock says.
"Certainly if you live in a city like Auckland, there's almost always some light, somewhere."
The British-born scientist has built his career on detecting the darkest corners of the universe.
Once a Westlake Boys High School student, his pursuit of one of science's greatest mysteries, "dark matter", has seen him progress to head of the Harvard-Smithsonian Centre for Astrophysics in Massachussetts.
His journey through Princeton University and MIT to Harvard began with a science degree at the University of Auckland, where he returned this week to receive a distinguished alumnus award.
Professor Alcock says the night sky may look crowded by stars and planets, but these make up only 5 per cent of the universe. Three-quarters of it is filled with dark matter.
Because dark matter does not emit or scatter light, it is not detectable through a telescope. But as the astrophysicist explains, there are other ways of finding invisible things.
Einstein theorised that the gravitational pull of a massive object, such as dark matter, can "bend" space. So any light passing through space also bends, and becomes more focused.
If dark matter passes in front of a star, that star's light appears brighter. By monitoring the brightness of stars, dark matter can be found.
Professor Alcock: "Supposing you were in a very dark place. And in the distance there is one bright light, maybe a streetlamp in the distance. And as you were watching it, it got brighter, then returned to its original level.
"You would wonder about that. There are two possibilities. One is that someone might have turned the streetlamp up, and down. But you think, 'That's never happened to any other streetlamp'.
"So you might think that something happened between you and the lamp. And that's what we're looking at - the massive, too-faint or invisible object that passes between us and the background."
While Einstein explained this phenomenon, he also predicted that it would never be seen because it was so rare. Professor Alcock, with the advantage of 60 years in technological progress, was more optimistic.
In the early 1990s, scientists could monitor only several hundred stars a night. In early 1993, Professor Alcock and a colleague engineered a system at Lawrence Livermore National Laboratory in California that allowed astrophysicists to monitor up to 30 million stars a night.
Eight months after his programme was launched, he detected amid screeds of data a star brightening over weeks, before fading again.
It was a slow-moving eureka moment, he says: "I never doubted for a moment we could do it." Over the next six years, 15 more events were observed.
The next step is finding what dark matter is made of. While Professor Alcock's focus is usually on the outer parts of the universe, he always has one eye on the Large Hadron Collider, where scientists are trying to recreate the elusive dark matter by smashing particles together.
He also is looking into the mostly unknown space beyond Neptune and Pluto, once thought to be the edge of our solar system.
Asked what the study of dark matter contributes to the real world, Professor Alcock responds: "Honestly, there is no practical application. What astronomy contributes is that sense of wonder that attracts a lot of people in the direction of science who might otherwise end up in other fields."
His first moment of scientific "wonder" came in after-school experiments in a Westlake Boys laboratory.
"We once set something on fire in a back room. It was a beautiful fire, zinc and something else. I can't imagine ever doing that now, with all the health and safety standards we have."
So, take note science teachers: that child who sets things on fire in class may well turn out to be the head of an Ivy League organisation.