Neighbors heard the explosion around 4:00 p.m. Tuesday in the 200 block of North 9th Street.
"I heard a loud boom and I saw some kids running," said one neighbor, Tra.
Officers say they even heard the explosion from inside the police station.
Chief Robert Smythe said even though it was made from a two liter soda bottle, and not very sophisticated, there is no question about what it was.
"It's a bomb and all the shrapnel goes out. In this case, it's plastic," said Smythe.
Police say the location where the bomb went off is very concerning. The alley backs up to Moose Field, part of Penn Middle School.
The explosion sent plastic and balled-up pieces of tinfoil some 20 feet away.
"If this happened and hour and a half prior, there would have been a number of kids walking down the street," said Smythe.
No one was hurt, but the group left a lot of evidence at the scene as they ran off.
Authorities believe that, based on what was left behind, the suspects learned to make the bomb online.
Anyone with information is asked to call Darby Police at 610-586-1100.