The Nature of the Dark Side originates from the novelization of Return of the Jedi by James Kahn:
He contemplated the dense impenetrability of the deep space beyond the window. Densely black as his soul – as if he were, in some real way, this blackness; as if his inner spirit was itself this void over which he reigned. He smiled at the thought; he was the Empire, he was the Universe.
This paragraph was the core of the poster, combined with one of the few feelings Palpatine can feel: the lust for power over all things (the other one would be rage). Therefore, the key messages were: total power and hopelessness.
Instead of pure blackness, I decided to use the galaxy on a black background and merged it with Palpatine to illustrate the sentence “he was the Universe”. Darth Sidious looks like a tumor; his presence distorts the spiral arms of the galaxy and replaces the brightest center of the galaxy with blackness and the Death Star. This is his true aim, the aim of the dark side: replace life with death.
There can be nothing outside his influence. That is why there cannot be stars on the black background: stars would mean there is life and hope outside. But, in this picture, there cannot be hope. The dark lord of the Sith controls everything, even the stars burn out.
The galaxy is blue because this color can be attached to him very well; just think of the Force lightning.
There are hidden figures in the spiral arms: his tools he uses to realize the total control. On one side, there are the disposable ones: Dooku, Gunray, Grievous, the clone troopers and the stormtroopers. On the other side, the more important servants: Darth Vader, Darth Maul and Tarkin.
The music that fits best to this picture can be found in the ROTJ soundtrack under the title Battle of Endor II (5:40-6:08 and 7:44-8:40), where the Emperor’s theme is played like a nightmare and then like a triumphant, dark hymn. You can hear this latter part in this video from 3:54.
I find this picture “terribly” appropriate because it conveys what I think about the ultimate coldness of the dark side (and is in contrast with everything I have faith in).