It isn't? Why? Space-Time is described as a continuim not a dimension and time is usually considered relative to gravity. Frequency is a measure of vibration and if two things, matter, universes , light, are vibrating at extremely different rates they are in effect invisible to one another which adds a dimension to their existance. For instance we can only see the 'visible spectrum of light with the unaided eye yet there is infra-red, ultra violet, etc. etc. wave lengths that are there also but we can't 'see' them. The same principle can apply to matter. One group of matter 'vibrating' at a frequency so different from another group would render them invisible to each other.