It's a tongue-twister, but yes, Medium has acquired MATTER. Both companies are relatively new, but Medium's co-founders Biz Stone and Evan Williams have a long history of success with such tools as Blogger and, most notably, Twitter. Medium, launched in late 2012, is a content creation and consumption platform, which is still invite-only.
MATTER is a longform journalism project, which got its start through a Kickstarter program in early 2012. According to the blog post announcing the acquisition, Medium founder Williams showed early support by donating to the cause.
MATTER assured its audience that, at least for now, nothing will change:
"We have no immediate plans to alter the team, the places we publish, or how much we charge for each article. More importantly, we have no plans - at any time - to stop crafting hard-hitting narratives about big ideas. We do, however, want to expand what we do, and Medium is a big part of that... We'll also be introducing some exciting changes at the MATTER website to make the site better for readers."