Any of a ubiquitous class of enzymes that catalyze the unwinding of double-stranded deoxyribonucleic acid (dsDNA) into single-stranded DNA (ssDNA). In all organisms (other than some viruses), genetic information is locked within a double helix formed by two antiparallel DNA strands. Although dsDNA is suitable for secure information storage, hydrogen bonds formed between complementary bases (Watson-Crick base pairing) impair readout of this information by the cellular machineries that frequently require ssDNA as the template. The unwinding of dsDNA into ssDNA that is catalyzed by DNA helicases is a function critical for virtually every aspect of cellular DNA metabolism including replication, recombination, and repair.