What the parent poster was asking - since this link is a single-post page, is what you're expecting to fill the space.

On the main stream, you have multiple columns of data. On a single post, what should be to either side?

Probably nothing.

You widen the single-post, maybe add inner padding, increase line-height, font-size. It just looks weird otherwise, the post is too narrow. Make it look like its comfortable in its environment instead of very shy and afraid to take any space.

I would prefer the margins narrow to no more than 10% of the page width. The left margin already has some navigation in it, fine, the right margin doesn't need to exist. If that means blank space below the article I'm ok with that. Having to scroll down to read an artificially skinny single post is, in my opinion, bad UX.

I find it difficult to read articles that are wider. There are a number of readability studies that back this up, it seems that 50-75 characters (including spaces) is a sweet spot for readability, and that things get terrible pretty quickly once you start increasing the line length. This design is 80 characters wide or so, putting it at the upper limit of reasonable width.

I would rather have to scroll down on an article that I can actually read, rather than fit the article on my screen but be unable to read it well.

Wide text only really works for very short text of a few lines. Long text comments on HN tend to suck pretty badly with wide browser windows.

Maybe ads? But I got adblock, so I don't know

You would rather see ads than whitespace?

Maybe recommended posts instead?

If I could take your suggestions in a slightly different direction, what I'm finding I prefer for textual content is:

1. Single-pane designs. Multiple columns, especially online, for main content text, is too hard to follow. In a cards-type presentation (e.g., G+) a 2-column display somewhat works, but 3 columns to me simply becomes noise. I think a multi-column (newspaper-like) display might work if you had explicit height alignments, see a site like http://freshnews.org, very old-school, but which essentially has a 3-column grid, and compare with, say, the NYTimes desktop layout. Note that new column heights aren't simply random, but tend to be lined up across the page, or at least a section of it.

2. A flexible main-body text width, but maximising at about 35-45 em. Look up "Edward Morbius's motherfucking web page" on CodePen.io for my interpretation of a popular meme, and pretty much how I style my own content. It's a very minimal ruleset. Pocket and Readability are good models.

3. A paginated, two-page-up mode, for desktop / widescreen mode. See many eBook readers or the Internet Archive's Bookreader in 2-up mode. This gives focus on a single page at a time, but the ability to reference across two pages. I use a tablet in portrait (1-up) mode a lot, and prefer being able to reference back and forth across two pages. A fixed pagination also feeds my spatial memory for where something was on a page. Pocket has a paginated mode, but it's dynamic, changing to wherever you've scrolled a page. I dislike that.

4. Except for the very narrowest display, at least a minimum margin on sides. I'll usually set 1-2em for even a small-factor mobile device, and prefer 4-5em for most widths. As text body reaches 45em, center the main body. Text that runs flush to the screen edge, or is sharply asymmetric on the page, is quite annoying.

5. Use those margins. I've written a sidenote style which puts annotations in the margin, with a dynamic repositioning for narrower widths (or rather the inverse, using Mobile First design). I also push illustrations / images into the right margin to allow text to wrap, but still leave a comfortable width, around inline images.

My problem with a "summaries on the side" display (sidebar) is that the summaries are so often entirely gratuitous and inappropriate. Single-column is far calmer. I move sidebars to headers and footers (through my own site-specific CSS rewrites), or simply disable them entirely.