By the time the attack goes forward, it's fully dark, but this actually protects the Southern troops from long-range artillery attack by the considerable gun line south of Cemetery Ridge.
The two battle lines crash together in the darkness and exchange fire at point-blank range. Casualties are extremely heavy on both sides, with bodies laying in heaps in the Woods, the Wheat Field, and around the Trostle farmhouse.
The defenders are forced back, but counter-charge and push the Rebs back in turn; re-taking the blood-soaked Wood and Wheat Field.
...only to see them come charging back out of the night.
The screaming rebs drive off the Union artillery and finally uncover the right flank of Sickles Corps. The union regiments fight hard, but are toppled like dominoes from right to left. Soon the entire center of the Union line is fleeing.
The casualties on this part of the field are 5,800 for the North plus approximately 1,000 prisoners for a total of 6,800 as compared to 5,000 for the South. Each combatant started with just over 15,000 effectives on the south end of the battlefield. Added to the casualties on the north end of the battlefield and the losses sustained on Day 1, the losses at Gettysburg were: North 18,000; South 12,000.