My son is almost 6 months old and when he gets mad and cries he has this vein the pops out of his neck and it gets dark. should i be worried?

Crying naturally increases the general vein pressure by backing up the blood flowing back into the heart. The effect is seen most above the breathing muscle in the upper half of the body. You are quite observant. It is natural and poses no risk to baby.

Likely not much to worry about if you only see the vein when he’s crying. During crying babies will temporarily raise the pressure in the veins causing them to become bigger and more noticeable. Blood in veins has a low amount of oxygen so it is expected to be “dark”. Bring your question up with the baby’s doctor at your next check up to make there isn’t evidence of an abnormal blood vessel.