The blob may have a giant black hole at its centre
It is a "Lyman-alpha blob" and is 55,000 light years across - as large as present-day galaxies.
Though younger such blobs have been found, "Himiko" confounds the idea that such large objects grew more recently by the merger of smaller ones.
The research will be published in the Astrophysical Journal.
Current cosmology models hold that between 200 million and one billion years after the Big Bang, the first colossal stars formed, emitting radiation that stripped light elements of their electrons and turned the Universe into a soup of charged particles.
Only after this "re-ionisation epoch" did matter as we now know it really start to clump together.
A number of theories of Lyman-alpha blob formation abound, but all are difficult to test
James Geach
University of Durham
Objects as big as modern-day galaxies should have taken significant time to build up from mergers of smaller chunks of matter.
So when a group of researchers led by Masami Ouchi at the Carnegie Institution searched among 207 distant galaxy candidates using the Subaru telescope at the peak of Mauna Kea in Hawaii, they expected to find smaller galaxies.
"We hesitated to spend our precious telescope time by taking spectra of this weird candidate," Dr Ouchi said.
"We never believed that this bright and large source was a real distant object."
Massive mystery
The team went on to measure the characteristic emission from hydrogen - the Lyman-alpha radiation of the name - and confirmed the blob was some 12.9 billion light years away.
They then trained the Spitzer Space Telescope, Very Large Array and UK Infrared Telescope on the object to determine how much mass is contained in the blob - or, rather, how much was contained 12.9 billion years ago.
Himiko - named after a mysterious queen of Chinese and Japanese legend - contains more than 10 times as much mass as galaxies of a similar age.
The blob was first noticed in data from the Subaru Telescope
"There are two possibilities: the standard scenario of galaxy formation is wrong, or this particular object is showing something unique," Dr Ouchi told BBC News.
A range of possibilities could explain the massive nature of Himiko: it could have a supermassive black hole at its centre, or perhaps it is a single giant galaxy with a large mass of about 40 billion Suns.
Dr Ouchi said the team was proposing observations using the Keck telescope in the far-infrared region to establish the rate of star formation happening inside Himiko, and whether star formation could lead to the "nebula" of ionised gas that accounts for its enormous size.
"Many early theories of galaxy formation predicted a Lyman-alpha 'fuzz' around early galaxies," said James Geach, an astronomer at the University of Durham who works on Lyman-alpha blobs.
"The problem is that no-one is entirely sure what mechanism gives rise to the extended emission; a number of theories of Lyman-alpha blob formation abound, but all are difficult to test," he told BBC News.
"We now have several good models that seem to do a good job of reproducing many qualities of the observable Universe.
"There is room for improvement however, and explaining these Lyman-alpha blobs is a prime example."
Both Dr Ouchi and Dr Geach agree that future "wide-field" searches that scan larger swathes of the sky are likely to uncover many more such extended blobs.
But, Dr Geach added, "until we know more about their physics, their connection to the formation of galaxies, and perhaps most importantly their lifetimes, we don't really know how they fit in".
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