Sed substitution did not go as planned...

Hello,

I am using Vim version 7.4.160 on AIX.

I opened my text file...
vim foo.txt

...and modified the first ten lines using Sed substitution...
:1,10 s/^/--/

After running this command the beginning of lines 1 through 10 do have the "--" correctly placed. However, the first character of each line in the file is now highlighted (as shown in the attached image). The pattern now recurs for any text file I open with vim.

Why does this happen? How can I "undo" this new pattern so that vim does not highlight the first character of each line?

Thanks,
Marcus Martinez

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