Datatype bigger than bigint
Numeric/Decimal, Float/Real. There's a great answer here, some official documentation here and here, and I also posted a few opinions ages ago, but here is a quick demonstration:

-- overflow error:

DECLARE @x BIGINT        = 9999999999999999999;

GO

-- works:

DECLARE @x DECIMAL(38,0) = 99999999999999999999999999999999999999;