All large hematomas pose some degree of danger. I'll share a tip I learned from an Indian national med student who helped me when I got hurt in crowded metro station. The fact he was Indian is meaningful - he said this was not taught to him in med school, but is a traditional first aid used in India for millenia. His formal medical knowledge helped to explain the reasonableness of it. I was running up a long set of granite stairs to catch a train. I slipped and fell face first against the hard edge of a granite step, right across my brow (supraorbital process). That should have made one fucked up ugly bruise. But it never bruised or swelled up at all, thanks to that guy. My brow was just sore for a couple days.
The med student was a few steps behind me, helped me up to the platform and immediately began massaging my brow slowly, from the impact area outward in a spreading motion, using medium high force with his thumbs, while he used his fingers to immobilize the sides of my skull, explaining calmly why and who he was. This method of massage presses the leaked blood cells out of the mass and spreads them out thinly, farther into the surrounding areas of undamaged endodermal tissue via the capillaries and through between the tissue layers. The leaked cells can then be more easily resorbed and carried away cell by cell, which is better than risking that a clot of them will break loose within a larger blood vessel that was temporarily squashed at the injury site.
Since then, I have massaged potential bruise injuries on my skin every time. No bruising. No swelling. Less pain for less time. I would not do this if you believe the bruise is very deep and may be involving a large vein or artery - that requires a doctor to rule out a deadly embolism (blood clot in large vessel - can cause stroke, sudden death by blocking pulmonary artery). So, if your neighborhood bully or a road raging fucktard tries to break your arm with a baseball bat, just elevate the arm and let the ER deal with it.
