Q: running very low on memory on macbook pro 15" retina 8gb even with all programs closed - what's up?

OS X 10.10 Yosemite

OS X tries to use all available memory before writing to disk, to make the computer run faster.

This type of RAM usage was introduced with Mavericks.  Yes, it is good.  RAM not used is RAM wasted, so if you have the OSX use as much as is available, greater efficiency will be achieved.  You will have to reorient you thinking about RAM usage. 

Ciao.

How to use Activity Monitor - Apple Support

you can learn more about what is running in the background and using memory and other problems with

http://www.etresoft.com/etrecheck