The James Webb Space Telescope has spotted six massive galaxies that emerged not long after the Big Bang, a study said Wednesday, surprising scientists by forming at a speed that contradicts our current understanding of the universe.

Since becoming operational last July, the Webb telescope has been peering farther than ever before into the universe's distant reaches — which also means it is looking back in time.

For its latest discovery, the telescope spied galaxies from between 500 to 700 years million years after the Big Bang 13.8 billion years ago, meaning the universe was under 5 percent of its current age.

Webb's NIRCam instrument, which operates in the near infrared wavelength invisible to the naked eye, observed the six galaxies in a little-known region of the sky, according to a study published in the journal Nature.

Two of the galaxies had previously been spotted by the Hubble Space Telescope but were so faint in those images that they went unnoticed.

These six new "candidate galaxies," so-called because their discovery still needs to be confirmed by other measurements, contain many more stars than scientists expected.

One galaxy is even believed to have around 100 billion stars.

That would make it around the size of the Milky Way, which is "crazy," the study's first author Ivo Labbe told AFP.

For this young galaxy to achieve the same growth in just 700 million years, it would have had to grow around 20 times faster than the Milky Way, said Labbe, a researcher at Australia's Swinburne University of Technology.

"According to theory, galaxies grow slowly from very small beginnings at early times," Labbe said, adding that such galaxies were expected to be between 10 to 100 times smaller.

But the size of these galaxies "really go off a cliff," he said.

The newly discovered galaxies could indicate that things sped up far faster in the early universe than previously thought, allowing stars to form "much more efficiently," said David Elbaz, an astrophysicist at the French Atomic Energy Commission not involved in the research.

This could be linked to recent signs that the universe itself is expanding faster than we once believed, he added.

This subject sparks fierce debate among cosmologists, making this latest discovery "all the more exciting, because it is one more indication that the model is cracking," Elbaz said.