The universe isn't a static, stale place. It's a whirlwind of organized chaos, constantly changing, growing, evolving, creating and dying. Galactic gain and loss on a hugely industrial scale is an everyday event for our universe.
We're unable to accurately study the outer reaches of the universe, so scientists have closely monitored the death and production of stars within our own Milky Way to form a model. Observing the conversion of masses of gas into stars, they have concluded that 275 million stars are born every single day in the universe. Rather grimly, this model is based around observable star deaths - the formation of new stars relies heavily on older stars going supernova, thus providing mass and heat to gas clouds which enables them to form stars of their own.