アブストラクト：
Our current understanding on the cosmic star formation history during the first two billion years (redshift z >3-4) is mainly limited to galaxies identified in the ultraviolet. A number of massive and rapidly star-forming galaxies that are completely obscured in the ultraviolet have been recently confirmed to be at z > 4-6, raising a concern that many massive galaxies may have been missed from previous ultraviolet-based studies. A complete census of this missed massive galaxy population is critical to constrain models of massive galaxy formation. However, most of previously confirmed cases are extreme and rare starbursts with star formation rates exceeding 1000 solar masses per year, and are very likely only the tip of the iceberg. Here we report the submillimeter (870um) detections of 40 massive, star forming galaxies at z > 3-4 that are unseen in the rest-frame ultraviolet, using only 2-minute integration with the Atacama Large Millimeter/submillimeter Array. With a space density of n ~ 10^{-5} Mpc^{-3} and star formation rates of ~200 solar masses per year, these galaxies contribute a total star formation rate density ten times larger than that from equivalently massive ultraviolet-detected galaxies at z >4. Residing in the most massive halos at their redshifts, these galaxies are the seeds of present-day's largest galaxies in massive groups and clusters. Such high abundance of massive and dusty systems in the early universe is unexpected according to current models, and challenges our understanding of massive galaxy formation.