[CentOS] Help with yum

I just did a fresh minimal install of centos 7 on new hardware. While 
playing around with window systems I removed X windows like so:

 >yum remove "X Window system"

and then tried to re install

 >yum group install "X Window system"

which gives this error:

Maybe run: yum groups mark install (see man yum)
No packages in any requested group available to install or upgrade

I tried running:

 >yum groups mark install "X Window system"

and get the same message. I tried different permutations removing the 
"s" on groups. Nothing seems to bring back the yum "X Window system" 
group. Can anyone help me resolve this issue and tell me what I did wrong?

Thanks for your help.

-- 
Paul (ganci at nurdog.com)
Cell: (303)257-5208