Description: Gravitational waves are disturbances in the fabric of spacetime and are generated by accelerated masses. Gravitational waves were predicted by Albert Einstein's general theory of relativity but were measured for the first time in 2016 by two LIGO and Virgo observatories. These first waves originated from two merging black holes 1.3 billion light years away. This cutting edge phenomenon can be used to introduce gravity as a fundamental force and to give a sense of scale of the Universe.
Description: This phenomenon can be used in elementary to explain darkness and illumination. Even though light travels between the Sun and the Earth space appears black because there is nothing (e.g. gas, dust, etc.) for the light to reflect off of. This can also be used as a much deeper phenomenon. Since space is filled with billions and billions of stars and each of them give off light why isn't space completely light? The answer lies in the expansion of the Universe.
Web Resource: Ask Ethan: Why does space appear black?