A virtually unknown U.S. professor has taken a major step in solving a numerical problem that has baffled mathematicians for centuries, experts said.

Yitang Zhang, a researcher at the University of New Hampshire who once took a job a Subway because he couldn't secure an academic appointment, has made a significant step towards settling a long-standing question about prime numbers -- numbers that can only be divided by themselves and by one.

An oddity among prime numbers is that they often come as pairs known as "twin primes" separated by only two, like three and five, 11 and 13 or 18,383,549 and 18,383,551.

Mathematicians have long suspected there is an infinite number of twin primes -- expressed in a theory known as the "twin prime conjecture" -- but no one has ever been able to prove it.

Zhang has demonstrated no matter how large a twin prime is, there will always be another pair of primes separated from them by less than 70 million.

While not proving an infinite number of twin primes, Zhang's work effectively proves the distance between prime pairs does not keep on increasing to an infinite size.

Members of the editorial board of the Annals of Mathematics journal, which published the work, said Zhang had published "hardly anything" before and was not regarded as a "big name" in mathematics circles.

"It's a steady stream of papers which tends to get you jobs," board member Richar Taylor told The Daily Telegraph. "Maybe [Zhang] likes to think about the big problems -- and you don't solve those very often."

Working at the sandwich shop while seeking a university position "wasn't bad," Zhang said, but "whenever I was doing it I was thinking about maths."