In Sunni Muslim Law, a marriage contracted without witnesses is

Under Sunni (Hanafi) law, a marriage must be contracted in the presence of two adult Muslim male witnesses, or one male and two female witnesses. A marriage without witnesses is irregular (fasid), not void, because the defect can be cured; an irregular marriage may be ended by either party at any time, and if consummated the wife is entitled to dower and the children are legitimate.

Under Shia law witnesses are not required, so such a marriage would be valid there.