While the human genome represents the most accurate vertebrate reference assembly to date, it still contains numerous gaps, including centromeric and other large repeat-containing regions – often termed the “dark side” of the genome – many of which are of fundamental biological importance. Miga et al. present the first gapless assembly of the human X chromosome, with the help of ultra-long-read nanopore reads generated for the haploid complete hydatidiform mole (CHM13) genome. They reconstruct the ~3.1 megabase centromeric satellite DNA array and map DNA methylation patterns across complex tandem repeats and satellite arrays. This Telomere-to-Telomere assembly provides a superior human X chromosome reference enabling future sex-determination and X-linked disease research, and provides a path towards finishing the entire human genome sequence.