A large (compared to Earth life) species of creatures live in space. They gain energy from interstellar dust and starlight. How they do this is a separate question so not relevant here. However they can adjust their metabolism to eat enormous amounts quickly when food is available or hibernate for thousands of years if it isn't.
The question is, how do they move around.
So far I have:
Solar sails. They spread their 'wings' and are pushed by radiation from nearby stars. They live virtually for ever, so time and speed aren't a problem.
They can store faeces indefinitely and when they expel them they do so in a controlled way that allows acceleration and braking and course changes.
They also 'swim' or 'fly' through space by flapping or moving their extremities.
According to Newton the third option (swimming) is impossible in a vacuum if it is done simply by using bodily movements.
So what discoveries in physics since Newton will enable them to move this way?
I'm thinking of the ideas that even a vacuum is not completely empty and that dark matter and energy exist. Also - Is there enough ordinary 'stuff' such as interstellar gas for them to swim through, perhaps even gaining enough momentum to travel from one gas cloud to another, drifting between them?
The creatures are completely flexible in the manner of an amoeba. They can assume almost any shape and can expand to a flat disk a few kilometres in diameter or contract to a ball shape.
I think I haven't made it entirely clear. While the creatures may have other methods of propulsion (and in fact they do as mentioned above), I particularly want them to swim through space as well. They can be seen majestically pulsing along looking a lot like some vast version of a sea-creature on Earth.