Split milk

My space obsessed 5 year old son was watching a Sci- fi movie with me. Post movie conversation as follows:

'There are real aliens right?'
'Well some think so ...'
'They look like the ones in the movie? '
'The movie showed imaginary .'
'I think there are aliens many galaxies away but they will never come to earth .'
'Why not? '
'There is a big problem. They think our galaxy - Milky Way- is just milk split in the sky - that's the problem. They may think it is stinky spoilt milk and never come near it. So there will never discover earth.' :(