Double helix DNA exists due to

Double helix DNA exists due to Hydrogen bonding between nitrogen bases, and base stacking interaction.
Double-stranded DNA consists of two polynucleotides that are arranged such that the nitrogenous bases within one polynucleotide are attached to the nitrogenous bases within another polynucleotide by way of special chemical bonds called hydrogen bonds.
Its double helix consists of two spiral chains of DNA. This double helix shape is often visualized as a spiral staircase.