Dark matter is material that is believed to form a large part of the universe, but which has never been seen.
N-UNCOUNT
Dark matter is material that is believed to form a large part of the universe, but which has never been seen.
N-UNCOUNT
Collins COBUILD. Толковый словарь английского языка для изучающих язык Коллинз COBUILD (международная база данных языков Бирмингемского университета) . 2012