On a typewriter, all the characters are monospaced; that is, they each take up the same amount of space – the letter i takes up as much space as the letter m. Because they are monospaced, you need to type two spaces after periods to separate one sentence from the next.
However, on a computer the characters are proportional; that is, they take up a proportional amount of space—the letter i takes up about one-fifth the space of the letter m. So you no longer need extra spaces to separate the sentences
Of course, this one-space rule applies just as well to the spacing after colons, semi-colons, question marks, quotation marks, exclamation points, or any other punctuation you can think of. This is a difficult habit to break, but it must be done.
The use of two spaces between sentences was correct in the days of the typewriter, and a lot of people still hold on to that technique. However, in this century it is not the correct method. Computers automatically insert an "m space" after a period, while "n spaces" are inserted between words.
The editor's handbook also clearly states: ONE SHORT SINGLE SPACE AFTER THE PERIOD BEFORE YOU START THE NEW SENTENCE.
Therefore, unless the "instructions to the author" for the journal you are submitting this paper to specifically states that you must use 2 spaces, then please use one space only.