If there is a fifth force of natures (I think there are more) then you are going to need a force carrier. Certain kinds of matter can interact with certain forces. Without that kind of matter around it's very hard to notice that kind of force around. This means that matter that interacts with different forces are blind to matter that interacts with different forces. The exception is gravity because gravity is really caused by energy density (for which matter counts) warping space-time and isn't really a force. If there is energy associated with the formation of that matter then it will have a gravitational effect. Thus we see that there is a whole lot of matter that we can't seem to interact with at all but still interacts gravitationally. My guess is it's not very uniform in it's nature either. The real fun thought is that we didn't observe electricity for a long time, but our bodies make use of it, and biology in general. In fact while our ancestors were ignoring electricity, for them to see the world around them and then ignore it required them to use electromagnetic effects to literally see the things they were ignoring. And then it required electricity to fire between synapses for them to process what they were seeing. Could biology that likes to take advantage of every kind of interaction it can access use physics that we don't understand? What would we have thought about neurons if our biology had gotten far ahead of our physics? Perhaps it couldn't no matter how much effort we had put into biology because by misunderstanding those neurons useful biology would be frustrated.