People were taught to use two spaces following a period in the days of the typewriter and those double spacers became parents, teachers, and professors who taught word processor/computer-era students to use two spaces after a period. It’s a vicious cycle.

I have even seen two spaces following periods in text messages – a form of communication where spelling, grammar, and punctuation are often completely eliminated!

It is a hard habit to break and though one space after a period is now considered standard, the most important thing is to be consistent.

MLA (Modern Language Association)
MLA Handbook FAQ – How many spaces should I leave after a period or other concluding mark of punctuation?

Since word processors make available the same fonts used by typesetters for printed works, many writers, influenced by the look of typeset publications, now leave only one space after a concluding punctuation mark.

Chicago Manual of Style
Chicago Style Q&A: One Space or Two?

There is a traditional American practice, favored by some, of leaving two spaces after colons and periods. This practice is discouraged by the University of Chicago Press, especially for formally published works and the manuscripts from which they are published.

APA (American Psychological Association)
The APA style guide is not available online, but I used this APA Style Essentials summary. One must use the Publication Manual of the American Psychological Association.

Spacing after Punctuation: Space once after commas, colons, and semicolons within sentences. Insert two spaces after punctuation ending sentences.

Please note: In the quote advising those following APA guidelines to use two spaces after punctuation ending sentences above, there are not two spaces after the end punctuation of the first sentence…

And finally, I have to give props to the OWL.