We do this on our instance but only with links leading to external sites. Just copy and paste the code below into the "At end of the BODY" Custom HTML section in the admin area. I gave two examples depending on what you you want to do.
AJS.$("a").attr("target", "_blank"); <-- if you want all links to open in a separate tab
AJS.$(".external-link").attr("target", "_blank"); <-- if you only want links to external sites to open in a separate tab
To piggyback on what Alex said, see Atlassian's Confluence documentation on How to Force Links to Open in a New Window.
You also may want to consider voting on the feature request mentioned on that page to add this as a native feature: https://jira.atlassian.com/browse/CONF-11194
Meanwhile, some Confluence plugins offer this functionality via built-in macros. For example, Brikit's Theme Press (disclosure: we are a Brikit customer which is why I know this) offers a Open Links in New Window macro (one of many included macros in that plugin - see the full list).
I found another way to do it since my company won't add even a free plug-in.
This one looks great: https://confluence.atlassian.com/spacedirectory/view.action
Edit the space stylesheet:
Space Admin > Stylesheet
add the following:
Hey there Community! My name is Vannya Vallejo, the Channel Communication Specialist at Atlassian and I want to help Atlassian users like you learn about our Solution Partners and how they c...
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