Should I use a single or double space after a period?

In a paragraph of multiple sentences, should there be one or two spaces after each sentence's period for readability?

The paragraph is typed in Arial font. It is size 11 font. The monitor it is being viewed on is Ultra 4k HD. The background is white and the text is black.

I recently read somewhere (with an unknown credibility) that a single space is better, and double space is just legacy style from typewriters. But I was taught to double space, and I personally think double space looks better.

The above text has one space, because that's what the iPhone does.