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.
"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."
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