Can a hematoma in the breast be dangerous?

It depends: how big? "hematoma" is a term that means a collection of blood within a tissue in the body, usually soft tissue like muscle, skin, or breast. Most often it just means a bruise, but it can also mean a collection or pool of blood within a part of the body. Usually bruises are pretty minor, but a large "puddle" of blood can be painful. If it's big enough, it can compress blood vessels or become infected.
Haematoma breast. Haematoma is a collection of blood ( bruise ) . Most of them get absorbed over time, if collection is too big there is chance of infection.