\qquad (double space of current character set size)
$$a\qquad~b$$
\_ (where _ is blank!)
$$a\ b$$
(whereas $$a\b$$ is not a valid filter expression since the blank space is missing; it is recommended to use the tilde ~ instead of the simple whitespace)
\hspace{n} ,where n positive integer (= n Pixels)
$$a~\hspace{30}~b$$
$$a~\hspace{15}~b$$
$$a~\hspace{2}~b$$
$$a~\hspace{1}~b$$
\unitlength{m}\hspace{n}, changes the default unit length (m=1px) to be applied
$$a~\hspace{2}~b\unitlength{10}~\hspace{2}~c$$
(second space is 10x2=20px)
Note: Simple blank spaces and tildes (~) are ignored by the TeX filter and don't produce any space. You must use one of the defined math spaces to get a visible (extra) space.