We had a query recently about a database that had a lot of objects starting with the name _dta_. We explained that these where hypothetical indexes and statistics that had been created by the Database Tuning Advisor. Most of the time the Database Tuning Advisor cleans up these objects on completion but they can be left over if the Database Tuning Advisor had been cancelled or something else happened whilst it was running. You can safely delete these objects after you check that they are truly hypothetical. For Microsoft SQL Server 2005 and later, check the is_hypothetical column of the sys.indexes catalog view, for Microsoft SQL Server 2000, use the predicate sysindexes.status & 0x20=0 to identify real indexes.

https://technet.microsoft.com/en-us/library/ms190172(v=sql.105).aspx