Black holes can actually lose mass. Stephen Hawking theorized in 1974 that black holes radiate small numbers of particles (mainly photons), a process known as “Hawking Radiation”. This “evaporation” process can lead the black hole to shrink over time and ultimately to vanish completely. However, it is a staggeringly slow process: it would take about 10^67 years for a black hole the mass of the Sun to evaporate, significantly longer than the 14 billion years the Universe has existed.
