The traditional pub/sub paradigm is inadequate for dissemination in mobile networks, since any node is able to publish content at any time, thus easily leading to congestion. Therefore, a dissemination paradigm where mobile nodes contribute with a fraction of their resources is needed through the use of opportunistic networks. Moreover, as shown in recent work, a suitable organization for data dissemination in mobile networks should be centered around interests. Thus, we propose an interest-based dissemination framework for opportunistic networks entitled Interest Spaces. We focus on the first step required for implementing it in real life: data aggregation. Furthermore, we propose a method for aggregating data from encountered peers, in order for opportunistic nodes to have an informed view of the network and to avoid storing excessive amounts of information or performing many data exchanges.