Google and its algorithm Tests with hyphens blanks underscores

Domain Name Basics

The only characters allowed in a Domain Name are the 26 letters of the standard alphabet, numbers 0-9, and hyphens (-).

A Domain cannot begin or end with a hyphen, but it can contain a hyphen.

The Internet Corporation for Assigned Names and Numbers, also know as ICANN serves as the governing body for Domain Name registration.

ICANN is currently allowing up to 67 characters including the .com, .net, or .org

The underscore and the space are not acceptable characters in a Domain Name.

Test results with hyphens, blanks, underscores.

Test #1 Keyword1 Keyword2: any order, and also Keyword not together.

Test #2 Keyword1-Keyword2: only results with Keyword in that order, found together and fused together Keyword1Keyword2.

Test #3 Keyword2-Keyword1: that order.

Test #4 Keyword1_Keyword2: results with exactly that term.

FINDINGS is a site I took on about a month ago and carried out SEO work on it. I am in the process of SEM for it. Nonetheless the site has moved 7 million places in the Alexa rank system in under a month. I have chosen this domain because it is has a hyphen in the domain and in its Meta title, keyword and descriptions tags.

Test #1: only results with Keyword in that order, found together and fused together Keyword1Keyword2

“meta learning” (Position 11 in Google)

Returned 660,000 results

Test #2: only results with Keyword in that order, and found together

“meta-learning” (Position 12 in Google)

96,700 results

In my opinion it matters more if you have your Title, Description, Keyword and body copy have hyphenated words than it does in your domain name.

My definitive answer:
An SEO person can optimise for hyphenated and non hyphenated words. Unless it is critical, and you expect users to type in Keyword1-Keyword2, I personally will treat it as two words.