Why Two Spaces isn't wrong after a period
There is a battle being waged, and most of the world is largely unaware. Every so often I come across somebody who argues, "The modern world agrees, only one space should come after a period." And they feel very strongly about this, arguing with the vehemence of a crusader!
I had one friend tell me that the consensus is in, and anything arguing aginst one space is like tilting at windmills. Seriously?
Many justifications are made for this position, usually revolving around typography, typewriters, courier font and the like. The problem here lies in the partial information being presented, salted with some opinion and personal preferences.
It's even more entertaining when people try to back it up with technical / software explanations. Trust me, as a software developer I can assure you it has NOTHING to do with software, and everything to do with academics deciding to get wound up about something that isn't that big of a deal.
There is a very good explanation (from archive.org) of all the details behind this at the Heraclitean River blog, so I will not dig too deep into the details, but I urge you to review that information if you feel strongly one way or the other.
Suffice it to say, the TL;DR: as long as layout has existed there has been, "additional space" after a period. This is a good thing as it helps us to denote the end of a sentence, especially if you are a speed reader (as I am). It makes it easier to read at speed. If you don't do this, then maybe you don't care.
As for the software arguments—there is nothing in software that cares one way or another. Software can figure it out and make it look good in any configuration. It just isn't that big of a deal. Of note, I typed this up using two spaces, and even three just to be wild on the prior sentence. Can you tell? No, because the software properly adjusted the kerning when it's displayed.
It's really only an issue while in the document-writing process. The document editor shows it to you so you can choose how you want it. Once it goes through a modern layout engine, it's adjusted regardless of what you've typed!
My suggestion is to simply use one or two spaces, whichever is your preference, but keep it consistent. And don't tell people to do it differently if they aren't doing it your way.
My preference is two spaces when I write.
Sidbear: If you use a modern word processor that supports regular expressions in a search and replace (such as LibreOffice or Google Docs), you can easily switch using a regex:
\. +
And replace it with a period and one or two spaces as you desire.
Stop making this an issue people. Please. Just because you like it doesn't mean other people are wrong.