I made a 400 GB partition for Ubuntu.
Why isn't the partion showing up in Windows Explorer? Should I be worried or is this normal?
Windows can't mount Ubuntu's EXT filesystem by default, so what you see is perfectly normal. Windows utilities can see the partition, but they can't interpret the files inside.
Generally, if people want to share files between Windows and Linux colocated on the same disc, they create a third, "neutral" partition in a compatible format: usually FAT. That way, both Linux and Windows can read from the shared space.