Don't trust anyone who is called SEO agency, good SEO is done only by using good content, design and understanding how to make this information available to search engines. We are just designers with SEO knowledge.
Attempts to improve rankings in ways that are disapproved of by the search engines, or involve deception. One black hat technique uses text that is hidden, either as text colored similar to the background, in an invisible div, or positioned off screen. Another method gives a different page depending on whether the page is being requested by a human visitor or a search engine, a technique known as cloaking.