Why does bump form on scalp after injury or blow. Is it dangerous?

When someone gets an injury on the forehead or scalp, a bump is formed, if the skin remains intact.

The reason for this bump formation or swelling is explained below. 

The skin of the forehead and scalp is adherent to the underlying bone and there is no space present between them. When a person receives a blow on the forehead or scalp, either by an object or falling on a hard surface, it causes rupturing of the blood vessels inside the skin. If the skin remains intact then the blood that oozes from the ruptured vessels has no place to go. It is collected under the skin and it creates a bulge, this bulge is called hematoma. It is not serious if it is formed outside the scalp and it resolves spontaneously within 2-5 days.

Is this bump on the scalp or forehead dangerous?

As it has been already stated that if hematoma or bulge is formed outside the cranium then there is no problem but If such hematoma or bulge produces on the inner side of the scalp bone then it may compress the brain tissue, in such circumstances, patient may present with nausea, vomiting, confusion, drowsiness, loss of consciousness and weakness of one or more parts of the body. In such cases, you need immediate hospitalization of the patient and evacuation of that collected blood by creating a hole in the cranium.