Help:Whitespace
Whitespace is blank space that appears on the published version of a Wikipedia page in any namespace. It is not occupied by any characters, templates, multimedia, or anything else. In other words, it appears white.
Whitespace is not always desirable. It can give the page an untidy appearance and make the article look altogether incomplete. But it cannot always be avoided, and there are times when it may actually come in handy when added in small amounts.
Causes of whitespace
Table of contents
On pages with multiple headings, a table of contents listing the headings is automatically generated. When this occurs, the table of contents box will appear on the left side of the page between the lead section and the first heading. There will be no text to the right of the table of contents, though an image or a vertical template, such as an infobox, will appear in the space to the right if formatting causes it to require that space. Still, no text will appear in the space in between; it'll be white. (As you notice, there is some white space between the TOC box and the image above).
There are various ways to modify the appearing of the table of contents. Entering the templates {{TOCleft}} or {{TOCright}} at the top of the page will produce a table of contents to the left or right that does share its lines with some text. The magic word __NOTOC__ will cause the table of contents to be altogether absent, which for some pages may be necessary.
It is also possible to produce your own table of contents as follows:
{| id="toc" class="toc" summary="Contents"
! {{MediaWiki:Toc}}
|-
|[[#Heading 1|Heading 1]] {{·}} [[#Heading 2|Heading 2]] {{·}} [[#Heading 3|Heading 3]]<br />
[[#Heading 4|Heading 4]] {{·}} [[#Heading 5|Heading 5]] {{·}} [[#Heading 6|Heading 6]]<br />
__NOTOC__
|}
In an automatically generated table of contents, there will be one listing per line. But in a self-produced table of contents, multiple listings can be condensed into one line, as much as space allows, using the line break (<br />) text to mark the end of a line. The drawback to this is that you will have to manually add, remove, or modify headings as they are edited on the page. Still, it'll reduce the amount of whitespace on the page.
Templates
Whitespace cannot easily be avoided when a page with little text, often a stub, contains an infobox or similar vertical template that is quite tall along with a navbox or similar horizontal template at the bottom, and the amount of text on the page takes up far less space than the template to the left. Even without a horizontal template, there may still be a lot of whitespace to the left of the template between the final line of text and the category box at the bottom.
Images
Sometimes, images can contribute to whitespace. But the size of images can be controlled, so if an image is causing there to be a lot of whitespace, it may be worth reducing the size of the image, even just a little, in order to fill more of that space with text.
On a short page, a single image may be to blame for whitespace. If the height of the image is greater than that of the text, there will be some whitespace at the bottom.
On a longer page divided by multiple headings, it may be desirable to provide a small amount of whitespace (via additional blank lines added simply by hitting "ENTER" on your keyboard) so the image does not protrude into the next paragraph. The caption below the image contributes to its height, so this will take the art of balancing the image size to these extra lines.
Many infoboxes have room for an image, so if an image is placed there, it'll add more height that hopefully you'll be able to match up with the text.
Invisible comments
Comments in the wikicode added by <!-- Comment --> can contribute to whitespace. Format the comment to avoid this.
Addressing the whitespace issue
Very often, when you have undesirable whitespace, the only way to solve the problem is to expand the article. But only sourced information that belongs should be added. Under no circumstances should you feel compelled to add unnecessary text to an article in order to eliminate or reduce whitespace. It is better to have a page filled with whitespace than nonsense.
Sometimes, a minor fix will help eliminate or reduce whitespace. This may involve adding or removing one blank line from some part of the page, re-ordering templates, or the use of a gallery for multiple images.