With the word cocooning we usually express a sensation of protection in the sanctity of your house or favourite place. A feeling of leisure and not having a care in the world. Ah... what a comforting thought that is...
So how on Earth did anyone attribute the nick "cocoon" to this pair of galaxies that are currently racing past one another? Possibly because the biggest one does resemble and insect's cocoon at lower magnifications, but that's as far as my imagination goes. NGC4490 (the big one) and NGC4485 are a pair of closely interacting galaxies. The little one's just passed the closest point of its incredibly fast fly-by which has left the bigger one significantly distorted. Well... "just passed"... we're talking about millions of years ago here but in astronomical terms that's "only just" of course.
It's not the first time that I've written about galaxies that are zooming past or even crashing into each other (see e.g. my video about a galactic tango here). This universe is incredibly big but gravity, being only the third or the four forces that govern it, still remains absolute. Also the Andromeda Galaxy will crash into our Milky Way in 5 billion years' time. But as I've explained, that doesn't need to mean doom. On the contrary, these close encounters stir the matter in those galaxies up, triggering a burst of new star formation. Look at NGC4490 and the many bright regions that I managed to distinguish. These are areas of new, very intense star formation and so the fly-by of little NGC4485 has brought uncountable new stars into existence and... probably... a lot of new life with them.
What a cocooning thought... :-)