It was a spark in the night. A flash of X-rays from a galaxy hovering nearly invisibly on the edge of infinity.

Astronomers say they do not know what caused it.

The orbiting Chandra X-ray Observatory, was in the midst of a 75-day survey of a patch of sky known as the Chandra Deep Field-South, when it recorded the burst from a formerly quiescent spot in the cosmos.

For a few brief hours on Oct 1, 2014, the X-rays were a thousand times brighter than all the light from its home galaxy, a dwarf unremarkable speck almost 11 billion light years from here, in the constellation Fornax. Then whatever had gone bump in the night was over and the X-rays died.