Ghost object ?

Hello @,
I have found this
did anyone checked this?
If you look in the file explosion1.blend, you will see a strange spherical object in the middle of the scene
a bomb I guess, but does someone know how to create those little fellas ?

The concentric circles around the object are just a visualisation of the Particles Force Field. This can be turned on in the Particles Interaction panel (right next to the Effect and Constraint panels in the Object buttons F7).