What's the matter?

A Bose-Einstein condensate formed from rubidium atoms held at very close to absolute zero behaves as if it has negative mass according to US scientists. We are all familiar with positive and negative electrical charges, but the notion of a negative counterpart for matter has until now remained a purely theoretical construct. Now, Michael Forbes of Washington State University and his colleagues have demonstrated a material that when "pushed" accelerates towards to direction in which that force is coming rather than moving away. This seems to contradict Newton's second law of motion, but in modern physics is perfectly acceptable given a negative mass as the team has observed.