The genomes of phages—viruses that infect bacteria—are largely composed of "dark matter": genes that encode proteins whose functions remain unknown. Less than four years ago, a team led by Prof. Rotem Sorek at the Weizmann Institute of Science identified a new type of protein within this viral dark matter and dubbed it a "sponge." Viral sponge proteins are porous and specialize in trapping molecules within deep pockets—much like a sponge that absorbs water.