I had dark purple hematoma on my belly, now the color is gone but there is a nudule or ball (around 1 cm) where the hematoma was. is it dangerous?

Will likely resolve: Clotted blood is likely the little ball that you are feeling under skin where the 'purple hematoma" occurred. This may persist for quite some time. Warm compresses with gentle massage may help speed up the resolution process. If persistent for more than 3-4 weeks, or if you are very concerned, consult doc. Good luck.

Bruise: You didn't say why you had the hematoma. Hematoma are collections of blood. The nodule if probably a little blood clot under the skin or an inflammatory reaction to the blood. Both are safe. Apply heat liberally (don't burn yourself). See you doctor.

No: No it is not dangerous.A small hematoma like the one you have, will resolve spontaneously. A large hematoma which is symptomatic or causing pressure effects sometimes can be drained surgically.