A notable similarity between the two pieces can be found by analyzing the mindset of the composers at the time they composed their respective works.
After enjoying success with his Symphony No. 1, Sergei Rachmaninoff went through a dark stage in his life. Symphony No. 1, which had been thought of as a large success, was now being ridiculed by critics. At the same time, Rachmaninoff was experiencing personal problems. The combination of these factors resulted in a state of depression for Rachmaninoff, which consumed several years of his life. During this period, he wrote virtually no music. Rachmaninoff was finally able to overcome his writer's block after receiving therapy from Dr. Nikolai V. Dahl, who placed Rachmaninoff in a state of hypnosis and was able to recover Rachmaninoff's confidence in his skills as a composer. The sign of Rachmaninoff's recovery was the release of his Piano Concerto No. 2. This work was reflective of everything Rachmaninoff had just lived through. Full of melancholia, the Piano Concerto No. 2 enables the hearer to enter a world of darkness. If is certainly more gloomy and pessimistic than Rachmaninoff's previous works.
Rachmaninoff's expression of emotion was not uncommon for the time, as the Romantic Era was witness to increasing attempts by composers to increase the emotion expressed by their music.
While composing Origin of Symmetry, the album in which Space Dementia was released, Muse was going through a comparable phase. In the years prior to the song's release, all of the group's members were relatively young; they were all in their early twenties. The band was touring more now that they were becoming commercially successful, and the band was opened up to different life experiences. Several of Muse's members,-- including Matthew Bellamy, who composes the majority of Muse's songs,-- were experiencing problems in their respective relationships. This was an evident influence on Space Dementia, which turned out to be one of the darkest songs Muse has composed up to date.
It is interesting to note the similarity between the two pieces, which resulted because of the composers' attempts to express dark feelings through their works. Both Rachmaninoff and Bellamy resort to using minor key themes to accomplish this. Specifically, Rachmaninoff composed his Piano Concerto No. 2 in the key of C minor, while Bellamy incorporated multiple minor chords into Space Dementia. They chose minor key themes, because these are commonly associated with sad, and other related feelings. The same is true for minor chords, which are associated with feelings of sadness, melancholy, and depression. Taking this into account, it is not a surprise that Rachmaninoff and Bellamy chose minor key themes.