An artist's conception of Dark Energy.
Imagine you toss an apple straight up into the air. Due to gravity, one would expect the apple to come right back down to earth. But what if it doesn't? What if, due to some unseen force your apple continues going up, at an accelerated rate, no matter how much gravity pleads and begs for the apple to come back down. Could this really happen? Could there really be "anti-gravity?" On the scale of the Universe, there is; say "hello" to Dark Energy.
In the most basic sense, Dark Energy is akin to negative gravity. Where gravity is attractive, Dark Energy is repulsive. Dark Energy causes the Universe to expand at an increasing rate. For example, to a viewer on earth, gravity would attract a distant galaxy towards Earth, but Dark Energy would cause the galaxy to move away from the Earth. Similarly, neither force can be directly seen. We detect gravity by observing the effect between two masses. We detect Dark Energy by measuring the expansion of the Universe through the comparison of standard candles.
Where is Dark Energy?
Dark Energy is everywhere.
If Dark Energy is causing the Universe to expand, where is it?
Dark energy is everywhere. Dark energy is thought to be an inherent property of space itself. However we don't notice dark energy mostly because it is an incredibly small amount of energy per volume. The effect is only seen acting on the universe as a whole, much like how we can feel a gust of wind, but cannot feel the individual particles in air.