DUBAI, United Arab Emirates — Auction house Sotheby’s Dubai has unveiled a diamond that’s literally from out of this world. Sotheby’s calls the 555.55-carat black diamond — believed to have come from outer space — “The Enigma.” The rare gem was shown off on Monday to journalists as part of a tour in Dubai and Los Angeles before it is due to be auctioned off in February in London. Sotheby’s expects the diamond to be sold for at least 5 million British pounds ($6.8 million).