I tend to avoid singletons too but there are a few objects that make the most sense as singletons and a central messaging system is one of them. Despite the rants I've heard, singletons are certainly much better than global variables/functions because you always have to deliberately address it (versus global values just magically appearing out of thin air).
In the end every message sender and receiver must have some common point of intersection, and it is much better to keep your objects decoupled by having a common singleton shared by everything rather than have each of your message senders directly know about the message receivers.
While I'm sure some other architecture could be devised for your event system, to me it seems like a waste of effort to over-think it, especially when using an event system is already a big win over not using one.
EDIT: As for your specific example of an explosion event dispatched on a periodic trigger, I would probably have the events dispatched in some other object (like the turret gun or whatever causing those explosions) and not in the central event system. However those events would still be dispatched to the central event system.