Have you always wanted to be a celestial object? In this interesting take on the "grow bigger" genre you finally can. You begin as a smallest of asteroids and have to hit other asteroids to grow in mass and avoid marauding space ships and anything too much larger in the process. You move with arrows or ASDW (I used keyboard) in a 2D universe, while everything else is dictated by gravity.
At some point you will grow into a planet. This changes mechanics: no longer will you try to hit stuff but instead try to make asteroids orbit your piece of rock, after which you can absorb them. Mechanics are different for each phase: asteroid, planet, star and black hole. Planet phase is separated into regular and life planets (all with shields, planetary cannons and spaceships).
Orbiting is pseudophysical: that is, not real physics at all. The objects will actually be attached to the orbiting system. You should know that this is not a real physics simulator, so this is totally fine. This makes for some truly bizarre possibilities: you can form multi-star ring systems with ridiculous numbers of stars and yet have aggressively orientated fully evolved killer planets orbiting the system.
Story and achievements provide some 10-20 hours of playtime within three missions and 2-3 challenge mini games for each phase and finally a boss fight. They often make clever use of the game's physics and mechanics. You can also stop anytime you want and the cloud will make sure you can continue from where you left, or if you'd like to, from somewhere entirely different. Playing is very enjoyable and the only time I remember it getting tedious was when I grinded my way to 40 stars supersystem. Fortunately, you can save your favourite configurations (like that one with super-multistar megasystem) to be loaded any number of times again (handy for achievements) and play with physics options if regular pseudophysics gets boring.
Once you're done with the universe simply grow your planet-eating super-heavy neutron star into an all-devouring black hole and consume the world in a Big Crunch. A fitting end to a crazy little game.